Modina

Interesting... but it's a bit useless when he doesn't mention the peak-power that he has achieved. I doubt it is more than 40 or 50W. The most interesting and professional build that I saw on Youtube is this 2KW project:

Perhaps I should rather shelf my idea of designing my own lithium charger.... 😂

Current alone cannot give you the SOC. Here is an illustration as to why this is so: Let's assume your car's starter motor draws 140A cranking current. As the battery voltage drops, it can no longer maintain the 140A and the current will drop to 120, 100, 90.... and the cranking speed will get slower and slower. You realise that you will not get your car started like this. You now want to disconnect the battery and carry it to your garage for charging, but you accidentally touch your ring spanner across to the other contact and you cause one hell of a spark that is so intense, it melts and pits a part of the spanner. You just short-circuited your half-empty battery and this caused a short circuit current of maybe 200A. Much more than the cranking current of your starter motor. The short circuit is effective at near zero volts, but the starter motor needs 12V. You must think of an MPPT doing similar work to a gearbox. It will take the engine speed and available torque, and select the right gear for maximum power transfer for the set of parameters at any instance (engine speed, throttle opening, car speed, car load, road incline, etc) Think of the engine speed as your voltage and the torque as the current equivalent in an electric circuit. A gearbox can really only modify the gearing which results in a speed change. But the change in speed impacts on the available torque. An MPPT will control the voltage, but it can indirectly increase or decrease the current by adjusting said voltage. Other than a gearbox, the MPPT will not only control maximum power transfer, but it has to do so in a way that protects the battery. At some point, the MPPT will throttle back, to remain within the (programmed) battery specifications.

I am not a battery expert. There are a lot of things one could do in firmware, if the battery was 100% known. But there are many types of lithium battery chemistries and the inverter is not always the sole overlord of the battery. For instance, an external MPPT could be connected to the battery as well, and the inverter would have no way of knowing. I am sure the controller will implement timing elements in the charging algorithm but this will only follow typical values. The tables and graphs you provided all show voltage defined charging stages. Battery voltage is the most critical charge and health indicator. Once you know the voltage, then yes, you could consider the current and time to calculate gradients, capacity and even the internal battery resistance if you so would wish. But without voltage measurements you would be totally lost.

No problem doing bulk water heating from solar panels, but silly to burn batteries. A hot water geyser has enough thermal mass to keep water warm enough for a quick morning shower, except for the Brady bunch, or 8-is-enough families. 😆

That is corruption. I am talking about sabotage.

Yes agreed. SA had 15 years of no-power training. In countries like Germany they are totally un-prepared for black-outs. I heard that German telcos had standby power for 15 minutes. This subject can be discussed and it is good to make some basic preparations, however, do not fall for this constant fear-mongering. The last time I fell for fear-mongering was during the start of Covid. I have decided that any newly announced end-of-world-disasters can be eliminated by simply switching off the MSM news. I will never, never again believe a word of the fake MSM spin. The Eskom problem is complicated and has many sources but purposeful sabotage is most likely the over-bearing one. I suspect it is 100x worse than what any minister will ever admit. Some of it might be attributable to sub-contractors and some to political party infighting. However, the largest portion is directed from over the waters, by 3-letter international NGO agencies. The recent ICC flip-flop has proven again that SA politicians are, like everywhere else, just puppets-on-a-string, controlled by the puppet-masters connected with the WEF and the Elites. Realize that we live in a fake world where everything is just another movie scene to brainwash the sheeple. Luckily, some people now finally wake up to what is happening.

The charge controller needs to keep track of the battery voltage as it's the only way for it to know the depth of charge/discharge. The exception being, if it can talk to the BMS digitally, but this is not possible in the older Axpert inverters. Yes, the voltage will settle over time, but real time measure is the only way during charging. I presume the settling behaviour to be known and predictable and could therefore be corrected for in the charge algorithm.

Thanks @Logic From the pictures in your link, it seems that Victron is using a real shunt and not a hall-effect sensor. Judging by the marking on the shunt, it seems to drop 50mV at it's max rating. 50mV is really a tiny amount, so Victron must have implemented very low-noise amplifiers to work within these small signals. Battery current will cause a voltage drop over any cables making accurate battery voltage measurement impossible. There are two ways around this, one is to use a so-called 4-wire interface where you use 2 thick wires to carry the current and 2 thin wires to only sense the voltage. Clearly this isn't done by any inverter. The second option, very likely to be implemented, is to stop the charging current for a few milliseconds and then measure the voltage, but any further battery load would cause measurement errors.

Ah, @Bobster. I get many things wrong. Including the dribble I wrote above when I said Americans can buy a 230V kettle and it will run at 1/2 power on their 115V. P=V^2/R, so if the American where to buy a 2KW 230V kettle and use it on his 115V grid, it would only run at 500W, a factor 4 difference. I think that because of the battery costs, the first rule is to make sure you run low power devices, even if that means shelling out some money to replace some equipment. The second rule is that if you decide to use lead acids, to overspec them as you say and recharge them as soon as you can, even if you think that the SOC is still high. The third rule, for small scale users with light/TV/laptop & WiFi requirement, is to try NOT to use/need an inverter, but run with 12VDC as much as you can. I know this has challenges regarding all sorts of non-compatible plugs and even supply voltages. For a person with some DIY capability and some basic electric knowledge this is doable.

Sorry about the double posting. Somehow the message got stuck the first time. 🙄

You could run the 4 fan motors in series if you place a 12V zener diode across each one. Then add a resistor or perhaps a LM317 to drop the 8 or so volts that the batteries are over 48V. The fans shown are rated at 120mA each. So I would use the LM317 (TO220 case) as a constant current regulator set to about 130 or 140mA. To be on the safe side, use 1W rated zener diodes. Use a 9.1 ohm 1/2W resistor to "program" the LM317 for a 137mA constant current.

You could run the 4 fan motors in series if you place a 12V zener diode across each one. Then add a resistor or perhaps a LM317 to drop the 8 or so volts that the batteries are over 48V. The fans shown are rated at 120mA each. So I would use the LM317 (TO220 case) as a constant current regulator set to about 130 or 140mA. To be on the safe side, use 1W rated zener diodes. Use a 9.1 ohm 1/2W resistor to "program" the LM317 for a 137mA constant current.

If you feel like a hot cup of coffee and use battery power to boil the kettle... OK, fine. But using battery power to heat en-mass (geyser water), with or without a heat-pump...... I am just speechless.

I will ask some stupid questions here. What is a SmartShunt? Does that make use of a real shunt resistance or does it use a hall-sensor to measure battery current? Having such a device, I presume you have wired it into place and kept it there. If this is a true shunt, it will steal maybe 100 or 200mV, maybe more at very high currents. Maybe the Axperts will stop the charging for a few milliseconds during battery voltage checks, but if not, then your battery would not attain the voltages that your Axpert thinks it has reached.

The control box has a power supply to power up your keypad. It also reads the new thermostat and houses a relay to switch the element on or off.

A 5KVA Axpert inverter by definition is a 48V system. So 2x 12V batteries are no good. For 48V systems you should rather use one or more 48V batteries than fiddle and trying to connect 4x 12V batteries in series. Separate batteries would require you to purchase a battery balancer which would cost another R 1000 or even more. I have no personal experience with connecting generators, but from what I hear on this forum, it is unlikely that your new inverter would accept such power. It has to do with the generator hunting and not keeping a constant speed when loads change abruptly. Altering speeds result in changing mains frequency and thus unstable 50Hz.

Yes, these sort of motor start capacitors are normally plastic these days. A capacitor is not a component that should get warm at all. Voltage fluctuations will definitely cause power variations, 20V fluctuations would indeed be noticeable.

Huh????? You have an awfully large thermostat. 😅 It would do nicely as a self-defence weapon. That clobbering tool is not a thermostat but a PTC element. The new thermostat is a very small device with thin wires that gets inserted into the hollow tube in which the old thermostat was mounted. If you want to fit that PTC element you would need to drain your geyser (at least partially).

Well, @frivan reported having used these 2x 70uF caps but he didn't say for how long. Him only getting just under 600W will keep the current on the low side. If someone wants to run the element at substantially higher values, say over 1KW, then I would strongly recommend using 4 caps. But this is only an educated guess. One should run such a setup for 20 minutes and then feel how warm or not these caps run. I wouldn't want them to run more than a few degC over ambient.

I was asked back channel to calculate the power that the 3KW element would see when using a 140uF series capacitor. I will calculate it here: A 140uF capacitor will have an impedance Z = 1/ [2pi.f.c] = 1/[2 x 3.14159 x 50 x 140E-6] = 22.736 ohms The impedance of the capacitor will form a voltage divider with the resistance of the element. We can calculate the new voltage that will be seen by the element thus: V_load = V_in x R_load / [R_load + Z_cap] where: V_load is the voltage over the element V_in is the 230V mains voltage R_load is the geyser resistance of 17.7ohms Z_cap is the impedance of the capacitor @50Hz, i.e 22.7 ohms = 230 x 17.7 / (17.7 + 22.7) = 100.77 V The power dissipated by the 3KW element is: P_load = V_load ^2 / R_load = 100.77^2 / 17.7 = 573.7 Watts

Don't even consider lead acid batteries. In 2 months time you will be very sorry, when your game stops 70 minutes into load shedding. You will need at least a 100Ah 12V Lithium battery that will cost you 6K +. Many small inverters don't have a build-in battery charger and those that do, will likely only have lead acid chargers. Buying a separate inverter and lithium battery charger will be way too expensive. Whatever you buy, make sure you have a proper lithium charger that can charge at least at 10A or you will take forever charging your batteries. A inverter-only will also not be usable as a UPS. Trolley inverters are handy but often have proprietary hardware with batteries of unknown form-factor. The hardware might be "hard-coded" for some weird battery size/chemistry, making battery replacements or additions difficult. This is especially true for the cheaper units. Buy a trolley inverter that uses a standard 1000VA 12V Axpert inverter, or buy such an inverter separately and wire it up yourself.

About capacitors and the ESR. I think it would be better to use 2 or even better, 4 or 5 smaller capacitors connected in parallel. For instance, using 4x 45uF caps in parallel will reduce the ESR by a factor 4. Many smaller capacitors will be able to handle high current much better than one single large capacitor. @markus_m2 When I was a high school kid my dad was very disappointed in me for not showing any interest when he worked on his cars, doing some basic maintenance. Years later, I completely overtook him, by overhauling a GTI engine completely, even opened parts of the K-Jetronic fuel injection components which where sealed and regarded as "unserviceable" items. Since then, my cars never saw the inside of a service centre. So yes, learning theoretical aspects when you do not have a direct application is boring. I have little interest in planes or tractors or even Ferraris, because I will never own, fly or drive them. I might be more interested in new BMW tech, because I might, in 6 years from now, be able to afford such a car, buying from the 2nd hand market. 😄 I know many engineers that are dreamers. Very good with theory and they can tell you everything about various fighter-jets, performance cars, etc. But often they don't know the difference between the radiator cap and the oil cap. A school friend's dad was a science professor and he was literally about to pour engine oil into the radiator, luckily my friend could stop him. 😅

Firstly note that the following discussion is only valid for resistive loads such as elements. This will not work when your load is inductive such as a motor or 50Hz transformer. To de-rate a heater element you could do so in three way: 1. Use a transformer to reduce the supply voltage. You would NOT use a transformer in the conventional sense, but use the secondary wired in SERIES to either decrease or even increase you AC voltage. (Wired in phase would add the transformer voltage, 180deg out of phase would subtract the voltage) This works well but is very expensive. 2. Use a simple diode, mounted on a heatsink. The diode will only pass every other half-wave. It results in pulsating DC. This is cheap to do. It has the disadvantage of not reducing the peak current but only reducing the average power. It is also 50%. Nothing more, nothing less. 3. By connecting a non-polarized capacitor in series with the load. I will cover (3) here: Let's assume you have a 230VAC 3KW element. We can calculate it's current as I = P/V = 3000/230 = 13.0A. We can also calculate the element's resistance: R = V^2 / P = 230^2 / 3000 = 17.63 ohms Let us assume you only want to run the element at 50% power. That means you would need to connect a capacitor in series that has a impedance Z (at 50Hz) that equals the element's resistance. So let's see what capacitance will give us an impedance of 17.63 ohms at 50Hz. Z = 1 / [ 2pi.f.c ] where pi = 3.14159, f = 50Hz and c= capacitance in Farads ... (1) We can re-write (1) as C = 1 / [Z.2.pi.f] = 1 / [17.63 x 2 x 3.14159 x 50] = 1.805 . 10-4 F = 180 uF So you would need a 180uF non-polarised capacitor rated for 250VAC. You could obviously connect a few smaller capacitors in parallel. If you do, you could add a switch to select more or less paralleled capacitors and thus have some pre-set power settings to select from. A small capacitor will have a high impedance and the element will see very little current. To increase the current, you need to reduce the series impedance and thus increase the capacitor size. Note that no electronic component is ideal. For instance, a capacitor is not JUST a capacitor. It has a certain internal resistance and even an internal inductance. We talk of a capacitor's ESR = Electrical Series Resistance. You get so called low ESR caps that are used in power supplies, etc. The ESR will cause the capacitor to heat up. Now, I have absolutely no idea how much ESR a motor starter capacitor has. Remember that motor starters only have to work for a very short time. I can't comment if a 180uF motor start capacitor would be happy to pass many amps of AC voltage on a continues basis. The capacitor might not be so happy and have a short life expectancy. One would need to try it and just check that the capacitor doesn't get too warm.

I disagree with @zsde. You would want to charge your 100Ah batteries at about 20 to 30A. 30A at 25V = 750W. A 15A plug in theory is good for 3.45KW. So if Eskom wakes up, and your batteries go on full 30A charge, you could still draw about 2.7KW of power on your multiplug "essentials". In reality this will be a bit less, due to various inefficiencies within your inverter. Even if you would double on your planned 700W maximum load, you would still be in safe territory. Just make sure that your mains lead is made from a heavy gauge cable.