I have a small off grid solar system:

12 x300W panels. The panels are wired in 6 strings of 2 each. The strings face different directions, east, north or west (this gives maximum hours of power rather than midday peak power). This setup is high current - low voltage (which I prefer as low voltage DC is not regulated and it’s much safer).

Inverter is a Axpert 5K (which is good for low voltage - high current)

Batteries are 4x mecer 200ah lithium second life.

Backup generator.

With judicious use of electricity the system works really well. My conundrum is that I want to increase my battery capacity to last longer during the miserable Cape Town winters, however mecer says that you cannot use their batteries in parallel and I cannot find an inverter that is capable of running two strings of batteries and suitable for low voltage - high current setup. Even two Axperts in parallel need to be connected to the same battery string.

Without rebuilding my entire solar system are there any clever solutions out there that don’t break the bank?

5 hours ago, Bran said:

This setup is high current - low voltage (which I prefer as low voltage DC is not regulated and it’s much safer).

How low is the voltage and how high is the current?? This sounds like you use this for welding 🙄

11 hours ago, Bran said:

My conundrum is that I want to increase my battery capacity to last longer during the miserable Cape Town winters, however mecer says that you cannot use their batteries in parallel and I cannot find an inverter that is capable of running two strings of batteries and suitable for low voltage - high current setup. Even two Axperts in parallel need to be connected to the same battery string

Yes the Mercer bms is not capable to handle parallel setup. You can run 2 seperate inverters by means of splitting your loads that is isolated from each other ( seperate live and neutrals). If you don't have space for more solar panels then also split your solar array between the 2 inverters 2S3P. For the second inverter you can then decide on different lfp battery pack. What is important is that each inverter has seperate loads connected that is completely isolated from each other as the 2 inverter outputs can not synchronize.

@tony Lampard Your post is not related to the op's question.

16 hours ago, Bran said:

My conundrum is that I want to increase my battery capacity to last longer during the miserable Cape Town winters, however mecer says that you cannot use their batteries in parallel and I cannot find an inverter that is capable of running two strings of batteries

Above is the crux of his question!!!

15 hours ago, Bran said:

I have a small off grid solar system:

12 x300W panels. The panels are wired in 6 strings of 2 each. The strings face different directions, east, north or west (this gives maximum hours of power rather than midday peak power). This setup is high current - low voltage (which I prefer as low voltage DC is not regulated and it’s much safer).

Inverter is a Axpert 5K (which is good for low voltage - high current)

Batteries are 4x mecer 200ah lithium second life.

Backup generator.

With judicious use of electricity the system works really well. My conundrum is that I want to increase my battery capacity to last longer during the miserable Cape Town winters, however mecer says that you cannot use their batteries in parallel and I cannot find an inverter that is capable of running two strings of batteries and suitable for low voltage - high current setup. Even two Axperts in parallel need to be connected to the same battery string.

Without rebuilding my entire solar system are there any clever solutions out there that don’t break the bank?

My 1st question would be are the batteries still under warranty?

If not I would parallel the Mecer with any other lithium that uses phosphate cell or even another set of Mecer. I do take into account the cost from @TaliaB and I base it on the fact that I have used my Mecer 200Ah in parallel with 2 other totally different batteries for 3 yrs. No problem. I do THINK the reason for not using the Mecer in parallel could be that during a specific fault condition the Mecer BMS might be at risk. Here I call on @TaliaB to share with us his insight.

I would just ensure that each bank must have their own suitably sized fuses and the main fuses between batteries and inverter.

I have to point out that although my Mecer could provide enough current in their early life they later still had a good capacity but had a high voltage sag during starting of a fridge. This I would take into account that one might not get the 5+ yrs of service prior to buying a 2nd bank of Mecer but look at another brand. My Hubble S-100 are performing much better under fridge high surge current after 4 yrs.

10 hours ago, Demo said:

How low is the voltage and how high is the current?? This sounds like you use this for welding 🙄

2 panels in series so max volts around 70-75V

Max power around 3000W

So max current going into the inverter is around 42A

<120A DC is ‘extra low voltage’. So I don’t need electricians ect…

The axpert 5k minimum solar voltage is 40V. I’ve not seen anything on the market that is even close to that. Probably because it’s cheaper to have higher voltage lower current.

8 minutes ago, Scorp007 said:

My 1st question would be are the batteries still under warranty?

If not I would parallel the Mecer with any other lithium that uses phosphate cell or even another set of Mecer. I do take into account the cost from @TaliaB and I base it on the fact that I have used my Mecer 200Ah in parallel with 2 other totally different batteries for 3 yrs. No problem. I do THINK the reason for not using the Mecer in parallel could be that during a specific fault condition the Mecer BMS might be at risk. Here I call on @TaliaB to share with us his insight.

I would just ensure that each bank must have their own suitably sized fuses and the main fuses between batteries and inverter.

I have to point out that although my Mecer could provide enough current in their early life they later still had a good capacity but had a high voltage sag during starting of a fridge. This I would take into account that one might not get the 5+ yrs of service prior to buying a 2nd bank of Mecer but look at another brand. My Hubble S-100 are performing much better under fridge high surge current after 4 yrs.

Thanks, that’s an option. I’m pretty happy with my mecer 200Ah, but I don’t use them at their full rated capacity of 43.2-56.4 which I think is a bit excessive. So hopefully still many years left.

1 hour ago, Bran said:

Thanks, that’s an option. I’m pretty happy with my mecer 200Ah, but I don’t use them at their full rated capacity of 43.2-56.4 which I think is a bit excessive. So hopefully still many years left.

Batteries not under warranty?

1 hour ago, Scorp007 said:

Batteries not under warranty?

No, but a new set would be anyway…

2 hours ago, Scorp007 said:

. I do THINK the reason for not using the Mecer in parallel could be that during a specific fault condition the Mecer BMS might be at risk

This is a Grey area as to why supplier doesn't permit parallel connection. It could be due to bms compatibility or due to the fact Mercer use pouch cells in these specific models or due to warranty issues( basically play it safe approach.) @Scorp007 If you have used them in parallel to the same cell count same chemistry lfp battery for 3 years without problems then maybe the Op can use a diffrent lfp battery in parallel. It would be much better $$$ wise to just add another 5kwh battery than 2 inverters with separate batteries.

2 hours ago, Bran said:

2 panels in series so max volts around 70-75V

Max power around 3000W

So max current going into the inverter is around 42A

<120A DC is ‘extra low voltage’. So I don’t need electricians ect…

The axpert 5k minimum solar voltage is 40V. I’ve not seen anything on the market that is even close to that. Probably because it’s cheaper to have higher voltage lower current.

My inverter minimum is 30V. Range is 30 to 360V. 18A max input

22 minutes ago, Denns said:

My inverter minimum is 30V. Range is 30 to 360V. 18A max input

What brand/model? Can it run parallel battery strings? 18A is a bit low though (my axpert max input is 60A).

I don’t mind having parallel inverters (redundancy if one goes faulty), but not all inverters can be in parallel and have separate batteries.

Edited October 18, 2025Oct 18 by Bran

2 minutes ago, Bran said:

What brand/model? Can it run parallel battery strings? 18A is a bit low though (my axpert max input is 60A).

I was only referencing your Minimum PV voltage where you said you don’t really see any below 40V.

It’s a MUST inverter. And yes it can run parallel battery strings like most inverters.

Can it run multiple parallel PV strings (I assume this what you actually meant). Yes and no. If it is panels like the 330 or 340W that are commonly around 8 to 9A Imp then yes. You can parallel 2 strings to one input. The bigger 400W plus panels no as the current would be a bit too much.

55 minutes ago, Denns said:

I was only referencing your Minimum PV voltage where you said you don’t really see any below 40V.

It’s a MUST inverter. And yes it can run parallel battery strings like most inverters.

Can it run multiple parallel PV strings (I assume this what you actually meant). Yes and no. If it is panels like the 330 or 340W that are commonly around 8 to 9A Imp then yes. You can parallel 2 strings to one input. The bigger 400W plus panels no as the current would be a bit too much.

Sorry, meant separate battery strings.

23 hours ago, Bran said:

• 12 x300W panels

• 6 strings

• east, north or west

• Axpert 5K

• 4x mecer 200ah lithium second life.

• Backup generator

• I want to increase my battery capacity to last longer during the miserable Cape Town winters

• mecer says that you cannot use their batteries in parallel

Hi @Bran

I assume that the batteries are something like "SOL-B-L-M200 Mecer Second Life LIFEPO4 Lithium Battery 12.8V 200A" and you have 4 of them connected in series. So it's 10kWh of storage.

Do I get it right?
If yes, then:

• These SOL-B-L-M200 are LFP 4S for 12V, 16S for 48V. BMS is just basic, nothing super-complicated and works without any communication with the inverter.

• So, just get any LFP battery that will be 16S for 48V, parallel it together with these four SOL-B-L-M200's and you're OK.

• The best would be to charge your existing battery to 100%, then disconnect and charge the new battery to 100%, then connect together.

• The new battery should be at least 100Ah x 48V (16S). Reason is, that if M200's BMS will suddenly disconnect, all the charge/discharge load of Axpert 5K will be directed to this new battery. Therefore, the new battery must be able to handle such a load. Adding 200Ah would be even better, of course.

23 hours ago, Bran said:

The strings face different directions, east, north or west (this gives maximum hours of power rather than midday peak power).

You did not asked, but I have to react anyway:

East+north+west is great for feeding the grid, for PV without batteries, PV with flooded-lead-acid battery (FLA charges really slow at 90%+ SOC) and/or PV with a very small battery.

• In all of the above cases, designers prefer to stretch the production hours, because the small, weak, or non-existent battery is unable to store all the energy produced during the noon peak. Therefore, it's better to sacrifice the peak and spread the PV generation over longer hours.

• This E+N+W arrangement works excellent over spring, summer, fall, but has very poor results in winter.

For a serious offgrid system in SA, with a large lithium battery, the best is to point all the panels to the north.

• Reason is that in the serious offgrid setup the spring, summer and fall are of no issue, but the winter is where you want to squeeze every watt of PV production, in order to survive.

• For offgrid that's designed to work during the winter it's battery bank has to be so large that it will be capable of running the loads for a number of dark days in a row.

• Therefore, it's best to squeeze all the winter PV generation (where north is the best) and store it in the battery in order to use it tomorrow...or day after tomorrow.

I am not pushing you into rearranging your panels, just want you to know that if you will gradually increase your battery to something like 50kWh or more, then the E+N+W orientation of panels will start to be the winter bottleneck.

Edited October 18, 2025Oct 18 by Youda

16 hours ago, Youda said:

I assume that the batteries are something like "SOL-B-L-M200 Mecer Second Life LIFEPO4 Lithium Battery 12.8V 200A" and you have 4 of them connected in series. So it's 10kWh of storage.

On 2025/10/17 at 6:02 PM, Bran said:

12 x300W panels.

I know that the question has been posed as "How do I increase my battery storage capacity," but I think it's also reasonable to take a step back and ask about the energy generation and demand first.

How much energy can 3.6kW of panels even be expected to generate in Winter in Cape Town? I just popped some values into the PVWatts calculator and assumed the best-case scenario of all panels pointing North. I know it's not the case here, we're looking at a East-North-West setup, but anyways.

For June the expected generation is 296kWh for the month, or average 9.8kWh per day. That's hardly enough to charge up the 10kWh of storage, and quite possibly not enough to cover consumption on top. If the real-life generation and consumption values are different, @Bran please share it, but I'm wondering whether adding a further 10kWh battery is even going to help. You'll likely be charging it from the generator all the time anyway and taking further round-trip losses.

You may have to step into the realm of demand-side management. Okay, I'm not talking about AI-controlled devices, not even the more pragmatic timer-controlled transfer of loads to the daytime. Rather, using efficient appliances like a heat pump or, maybe do more cooking on gas or coals during Winter, defrosting your meat overnight in the fridge, turn your electronics off at the wall plug, or whatever you can to take load off from the inverter/battery system. If you can't add batteries or don't want to change the inverter, maybe a solar water heater will help.

That's true.
There always has to be kind of balance between the loads, production, panels, batteries and their C-ratings. Makes no sense to have batteries so large, that you will not be able to charge them within a week 🙃

On the other hand, from my experience, most of the offgrid PV systems have their batteries as the weakest link. Which is obvious - batteries were extremely pricey in the past. Now, while still not really cheap, they are affordable at least. So, for a lot of people it makes sense to upgrade their batteries now and then add more PV if they realize that their current panels are not enough to support the loads plus charge the batteries in a reasonable time.

Well, it's the neverending story... "a solar itch" one would say 😉

Edited October 19, 2025Oct 19 by Youda

On 2025/10/17 at 6:02 PM, Bran said:

Batteries are 4x mecer 200ah lithium second life

Am I right that these are 12.8V batteries. So you 800a * 12.8 = about 10kWh of battery?

I have 10kWh of battery, and it's a cold day in hell when I have 50% left as the sun is coming up. And that includes running the heat pump in the morning because I like a morning shower.

So I'd look at two things
1) Beefing up the PV. I had 12*325W panels - 6 N and 6 E. I am short of space on the N facing roof. I put 5 625W panels on the N roof. Moved 5 of the old panels to the E string (lots of space there) and the installer took the 2 left overs as part payment. This gave me way more PV power, on sunny and on overcast days. I could always charge my battery on a sunny day, but struggled when it wasn't clear and bright all day. Now I have no such worries AND I have more flexibility in the day. EG I used to avoid running the dishwasher after 10:30 because that's when the pool pump started up. Now I don't worry so much about that. Panels are cheap compared to batteries, so this need not break the bank.

2) Reduce your night consumption. I have a 10kWh battery. Usually when the PV starts turning on (and this is after I have run the heat pump) there will be > 50% available. So we use about 4.5 kWh over night. I just checked my system for the solstice this June, and we used 4.7kWh from 16:15 on the 20th to 7:15 on the 21st. If you can get somewhere near that (assuming battery fully charged at 16:15) then you will get through the night easily, have something in hand, and only have to replenish about 5kWh during the day. So you can look at load limiting, running things on timers so that they can't run at night and draw from the battery.

Some things in my favour: Full gas stove, heatpump for water heating. All appliances are fairly new and low consuming (our new freezer uses literally 1/3 of the appliance it replaced).

We are two mature adults. Routines pretty static, and we learned years ago already to go easy on the battery at night. This is easy with two pensioners. Might not be so easy with more people in the house and/or wider age range.

Edited October 19, 2025Oct 19 by Bobster.

We are pretty good consumption wise. I’ve never measured it exactly but around 4kWh max 5kWh per 24h day.

• All cooking and water heating is gas.

• Low wattage lights everywhere.

• Laundry and vacuuming is only for sunny days.

• No pool or anything like that.

• Hairdryers are banned 😄

Biggest single consumption is fridge and second is pressure pump (water is also off grid). These are also the two things you cannot really switch off if the weather is bad either.

I’m not adverse to adding a few more panels, but for for now they seem sufficient. On a sunny winter day they easily charge the batteries from empty to full. It’s the week long rains that are the problem and then more panels won’t help much anyway.

The problem I have is just how to add more batteries to the axpert - or an alternative inverter that can handle dual batteries, that has a PV input voltage of < 70V. Otherwise I have to rewire the entire system.

29 minutes ago, Bran said:

The problem I have is just how to add more batteries to the axpert - or an alternative inverter that can handle dual batteries, that has a PV input voltage of < 70V. Otherwise I have to rewire the entire system.

There are some inverters with dual battery input, for example certain Deye models can do that. But in your situation it does not make any sense to go this way. One of the advantages of your current 48V system is, that you can add more batteries in parallel as you go, without any hassle.

2 hours ago, Bran said:

I’m not adverse to adding a few more panels, but for for now they seem sufficient. On a sunny winter day they easily charge the batteries from empty to full. It’s the week long rains that are the problem and then more panels won’t help much anyway.

I made the changes I did because, like you, I can do it all on sunny days, but overcast days were not so easy. On sunny days the battery charges real quickly then after that I am just servicing loads and not using what the panels can produce. But on overcast days I cope better (or my inverter does).

2 hours ago, Bran said:

I’m not adverse to adding a few more panels, but for for now they seem sufficient. On a sunny winter day they easily charge the batteries from empty to full. It’s the week long rains that are the problem and then more panels won’t help much anyway.

Well that's it. I've given my system more woema (as us scientists say) but there is still a limit and if there are enough overcast days something has to give.

I was going to say something about having to reprogram myself to not be expecting a power outage, but this is no help to you. I can fall back on the grid as a last resort. You can't.

I do think you need to get more PV woema (as us scientists say) for the reasons that @GreenFields has given. You have to charge those batteries at some point, and it's preferable to do that for free. Check out prices, but I think you will find that good quality, higher output panels are not so expensive right now.

I agree with the last 5 comments that you should maybe look at increasing your PV. I generate about 3x what my house needs at the moment, and designed it so because I am going off-grid in the next month or so.

The primary thing, in my opinion, that people forget is that when it rains, there are periods of thick cloud cover where there is little to no generation, and then there is some light cloud cover and sometimes the sun may creep out for an hour or so and disappear. A larger PV array will extract way more power during those intermittent times the sun shows itself through the clouds.

Before May this year, I only had 3.25kWp of panels. And I struggled with just 1 day of cloud cover. With no changes to the battery bank, I added 2 more panels to push my kWp to 4.55. I also added 1.34kWp to take care of my geyser and not have it on the inverter.

Because of the above changes, I kept the breaker feeding my inverter off since May this year. Batteries have never dropped below 65% and I have since reconfigured and removed the AC IN cable for the inverter from the DB board, and have only the essentials and non-essentials on the DB board. My main outside breaker for the house is off and hasn't been turned on since.

I am currently 2 days in with rain in JHB, and my batteries are at 95% as I type this. But then again, I am in JHB and the weather isn't an exact match to CPT, so even when it rains, there are brief moments where even when the sun isn't completely out, there is decent generation from the panels.

My point still stands, though. I struggled with 3.25kWp, even though it met my needs completely and a bit more during sunny days. But it goes downhill very quickly when clouds show up. A problem I have not had since adding more panels (My batteries hit float at 9 am, and the rest of the day the panels are idling). And I am adding an additional panel to a PWM charge controller I had lying around to bring the panels for the battery up from 4.55kWp to 5kWp.

Guess what is more expensive than paying the price for 1 panel (1.1k to 1.3k rand these days), paying about 15 to 20 rand a kWh running a generator, which adds up to a couple of thousands a year.

I did my tests this year and have been satisfied with my system enough to know I can go off-grid since I had zero issues in winter and currently have had no issues with the rain. I am so happy with it, I am not even sure I will buy a generator even though everything is wired in and ready for one if the need arises.

Hi @Denns , something off-topic. I had that same SUN 30Ah pwm charge controller, actually i had 2 of them, was quite cheap also at the time. Just be very careful with them - both lasted about 6 months and both took the battery commented to it - with it to battery heaven! Maybe my cables was to thin - installer said it was fine? Just keep a hawk eye on them 🤞 When my second one also gave the magic white smoke, i decided - never again a pwm controller.

50 minutes ago, Demo said:

Hi @Denns , something off-topic. I had that same SUN 30Ah pwm charge controller, actually i had 2 of them, was quite cheap also at the time. Just be very careful with them - both lasted about 6 months and both took the battery commented to it - with it to battery heaven! Maybe my cables was to thin - installer said it was fine? Just keep a hawk eye on them 🤞 When my second one also gave the magic white smoke, i decided - never again a pwm controller.

Thanks for the information. It will have only 1 panel connected to it, so hopefully I won't work it too hard. The maximum voltage it should see is 39V and 13A current, which is basically half its spec on both fronts.

I don't think it has enough oomph to do any real damage to my large LA bank, but I will definitely keep an eye on it. I had it lying around, so I decided to just add it in since I had extra cable, breakers and a dB box. It is wired with a 10mm2 cable to the battery bank busbars.

Edited October 20, 2025Oct 20 by Denns

11 minutes ago, Denns said:

Thanks for the information. It will have only 1 panel connected to it, so hopefully I won't work it too hard. The maximum voltage it should see is 39V and 13A current, which is basically half its spec on both fronts.

I don't think it has enough oomph to do any real damage to my large LA bank, but I will definitely keep an eye on it. I had it lying around, so I decided to just add it in since I had extra cable, breakers and a dB box. It is wired with a 10mm2 cable to the battery bank busbars.

Will actually buy a Victron 75/15 charge controller. They are 700 rand. Will let the sun run for a few months and replace it even before it fails. The victron unit is MPPT also so will get some 20% more than what the PWM can do.