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Gel battery voltage plummets out of nowhere


Lee2

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On a 2kw Axpert I have two gel batteries in series, 120a each.

With a puny load of 100w after an hour or so the voltage plummets out of nowhere from 24.6v to 22.4v and the low voltage alarm starts and shortly after the inverter switches off.

If I turn the inverter off a multimeter shows the voltage goes back to 24v, turn the inverter on and back down to 22 we go, confirmed by a multimeter. These batteries are 3 months old, are gel batteries really that useless or is something wrong here?

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Lead acid sucks big time ,you only need to discharge them too far a few times and that is what you get. Nobody should be buying anything other than LFP batteries anymore. Its also possible that they are out of balance. Try disconnecting them and charging them up individually and then back in series and try again.

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2 minutes ago, Nexuss said:

Lead acid sucks big time ,you only need to discharge them too far a few times and that is what you get. Nobody should be buying anything other than LFP batteries anymore. Its also possible that they are out of balance. Try disconnecting them and charging them up individually and then back in series and try again.

This is at my folks house and in theory there is no way they have ever. been discharged by more than 30% given it is lights and bedroom plugs only

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2 minutes ago, Lee2 said:

This is at my folks house and in theory there is no way they have ever. been discharged by more than 30% given it is lights and bedroom plugs only

haha thats kind of the problem . You dont really know ,its very easy to discharge more than you think you are , especially if your low voltage cutoff is not set correctly or the inverter is not disconnecting from them,22v is very low. 

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3 minutes ago, Nexuss said:

haha thats kind of the problem . You dont really know ,its very easy to discharge more than you think you are , especially if your low voltage cutoff is not set correctly or the inverter is not disconnecting from them,22v is very low. 

i was watching it and it went from 24 to 22v and entirely skipped 23v. No idea why it is not cutting off, i guess that is another question on the inverter sub 

I got a sonoff 63a smart meter on their db that i periodically monitor so I have a very good idea on their habits so I am 99% certain this is not from over discharging

Edited by Lee2
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5 minutes ago, Lee2 said:

i was watching it and it went from 24 to 22v and entirely skipped 23v.

At that point the batteries were basicly empty,hence the extreme drop. Probably not the first time it has happend if they are dying after 3 months.

state-of-charge-chart-for-24-volt-agm-battery-packs.png

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13 minutes ago, Nexuss said:

At that point the batteries were basicly empty,hence the extreme drop. Probably not the first time it has happend if they are dying after 3 months.

state-of-charge-chart-for-24-volt-agm-battery-packs.png

The first time there was beep due to low voltage was about 3 days ago

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Five minutes later the batteries are supposedly fully charged now at 26.4v, confirmed with a multimeter, I disconnect the AC in and turn off AC out so load is 0% and the voltage drops to 25.8 in minutes with no load

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Lead acid and gel batteries are the worst option for renewable energy installations. They need to trickle charge up to 24 hours to regain full capacity. With loadshedding up to 3 times a day, those batteries can never be fully charged. They are not designed for renewable applications and do not last. Once you have a waterfall drop in voltage on those batteries, that is their limit. They are done. Throw them far away and invest in Lithium iron  batteries. I know they come at a cost, but that is unfortunately the only route to go. 

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My gels also dropped  to 25.8 after float charge from solar for the day with no load so that's normal . 22v cut of is very low for only constant 100 w it should be more around 23.5v 

The higher the constant load the lower the cut of volts . 

That chart is no load volts . 

What is your bulk charge volts it should be around 28.2 volts and float charge 27v . Even with 28.2 bulk charge the battery will draw high Wats at 27v float for a few hours before fully charged . You won't see Wats draw of you charging from grid but if you charging from solar you can then see the Wats drop from pv as the gels reach there full charge state , an gel at full charge on float charge normally draws a constant around  150w from pv this is the main problem with gels with with lots of load shedding as they never really reach full charge.

Install solar to help get the gels to full charge through the day and set setting to charge from solar through the day and from grid at night 

You could have out  of balance battery or just a bad battery check them with multimeter .

If you have only 2 120ah  in series then with 100w load  you should get about 13 to 14 hours depending on how efficient your inverter is,  if you not then most probably bad battery 

Lithium is much better. 

Edited by GMAC
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9 hours ago, Lee2 said:

On a 2kw Axpert I have two gel batteries in series, 120a each.

With a puny load of 100w after an hour or so the voltage plummets out of nowhere from 24.6v to 22.4v and the low voltage alarm starts and shortly after the inverter switches off.

If I turn the inverter off a multimeter shows the voltage goes back to 24v, turn the inverter on and back down to 22 we go, confirmed by a multimeter. These batteries are 3 months old, are gel batteries really that useless or is something wrong here?

For me there are a few things at play. Did you check what the voltage was vs hours the inverter is on during the 100W draw when they were new? 

Always a possibility that these batteries were old when bought and never close to 120Ah capacity. 

Using the capacity and not real voltage (SOC) during discharge can make one think they were never lower than 70% SOC. 

 

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21 hours ago, Scorp007 said:

For me there are a few things at play. Did you check what the voltage was vs hours the inverter is on during the 100W draw when they were new? 

Always a possibility that these batteries were old when bought and never close to 120Ah capacity. 

Using the capacity and not real voltage (SOC) during discharge can make one think they were never lower than 70% SOC. 

 

No I never looked at the voltage before i always just assumed there was tons of leeway due to the small load and the batteries should last years from the DOD charts i saw

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On 2023/04/13 at 9:26 AM, Lee2 said:

... With a puny load of 100w ...

100W at 24V is about 4A. Not a big number, but I won't call it puny by any account.

(I make assumptions below, reader beware)

Many lead acid style batteries that I see used in these setups are rated at 0.1C over 10h. I assume your battery is the same So for a 120Ah battery that's 12A. i.e. if you draw 12A for 10 hours, you'll get your 120Ah. The higher the current goes, the lower the effective capacity of the battery will be. 4A is 30% of the batteries "rated" max output. Obviously, it can go MUCH higher than that, but this is where the battery is rated at and this is important.

On 2023/04/13 at 9:35 AM, Lee2 said:

This is at my folks house and in theory there is no way they have ever. been discharged by more than 30% given it is lights and bedroom plugs only

Herein lies the rub. You say in theory, yet you don't know for sure.

This is used in their bedroom, for lights and plugs. I assume you have a mother. And I assume she likes to blow-dry her hair at least once every two days. A hairdryer can easily exceed 2kW. Let's assume your mother has a smaller one (1.8kW), in order not to exceed the inverter limits. 1,800W @ 24V is 75 Amps. That's a big number. That will absolutely decimate the batteries in less than 2 hours, if drawn continuously. 

I suspect the batteries are pooped, plain and simple. With the loadshedding that we've been enjoying lately, it's possible these batteries were cycled 2-4 times a day (even if that cycle was just down to 70%. That's almost 300 cycles over the last 3 months.

These gel batteries are quite rubbish, plain and simple. The people that build proper systems with these batteries use massive banks of the stuff connected in parallel, so that you reduce the effective charge and discharge current per battery significant. 

I had a UPS in my office to survive loadshedding until before my big-boy solar setup. I had a fancy battery readout connected, that gave real-time current readouts. I was shocked by how much something as silly as an Xbox would increase the current drawn from the battery (20-30A in my case, 12V system). It's easy to to forget that you current goes up 10X when you go from your 240VAC to 24VDC. Silly things like hair dryers, a heater, a television, a beefy laptop charger, or whatever, can really kill a gel battery quick sticks.

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Having said the above, take out the batteries, charge them up fully individually, and test them individually with one of those 12V battery test jobbies. It should be able to maintain it's voltage at a modest current.

One bad cell out of the 12 will kill the entire bank. Maybe you can salvage one and replace the other one.

Also make sure your connections are proper tight. It's possible that as the inverter heats up, there is a voltage drop over some connection, leading to your problems.

Measure the voltage on the batteries themselves, on the inverter's connection points, and across each cable run. Maybe you have an unacceptable voltage drop over your wires.

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