Battery consumption

Hi Guys,

I have 24 X 170ah SLA batteries. ATM all i am running is two fridges and one freezer. During the afternoon all batteries are fully charged. Within 3 1/2-4 hours (no sunshine), the batteries are reading 12.25 volts. Now this is under a little load. Switch off inverter and they will read about 12.7V. Now I know that they will always read less while under load but is this detrimental to their health? Also it seems like quite a large consumption considering there is 4080 ah of storage?

Thanks Guys.

You harm them when you operate them outside their specifications or take them below 50% SoC.

Voltage to SoC relationships are ONLY valid when the system has been completely at rest for several hours - no charge or discharge. 12.7V unloaded is a relatively high state of charge.

0.45V instantaneous drop under "little load" may indicate you have a bad connection somewhere between the inverter and batteries. Recommend you start at one inverter terminals and check all connections in the series circuit for correct torque until you get to the opposite inverter terminal - including battery interconnects. Ensure all crimps/terminals are of high quality and no heat shrink is caught between contact faces.

Insufficient wire gauge and low quality breakers can also contribute to voltage drop.

How old are the cells and how often are you checking the individual batteries health?
It would not be uncommon for a large bank to have a couple of unhealthy cells and they would drag down the performance of the entire string.

I'd start by checking water levels in each one and make sure you have them all topped up correctly when charged.

I suggest you evaluate the performance of each battery on its own. Unless the batteries are resionably new and have been charged correctly you don't know the actual capacity. My guess is that a number of your batteries are at the end of their service life.
Having a resionable charge current for over 4000Ah requires a sizable charger and cable setup, that seems impractical at 12 volts.

Mike

Lead acid batteries have a large overpotential voltage which is the voltage slump overhead required to support demanded cell current.

Starting from a full state of charge with rested no load voltage of 12.7vdc, if a 25% of 20hr discharge rate AH rating in C(A) load (25A for 100AH battery) is placed on battery, the voltage will slump to about 12.0vdc in about 20-25 minutes. This is equilibrium to support 25% C(A) load rate.

After 30 minutes with that 25% C(A) load you would have consumed only about 13% of battery capacity even though terminal voltage is showing about 12.0vdc. If you then remove load, within 15-20 minutes the open circuit voltage will recover to near the full voltage level, between 12.6v and 12.7v.

Lead acid battery chemistry has a lot of chemical conversion processes going on converting several compounds to other compounds. All these chemical conversions take a lot of overhead energy to drive the chemical processes. This makes their energy conversion efficiency moderately poor. Li-Ion chemistry is much more efficient, Nickel-iron (Edison cells) batteries are even worse efficiency than lead acid batteries.

Battery condition impacts the amount of overpotential voltage slump. With 24 batteries in parallel there is likely a lot of load sharing variation between batteries. I would guess some of the batteries are not in too good condition. You can load test each battery by itself to check its condition. I use a small 300 watt inverter loaded with a few light bulbs to load test lead acid batteries in 100AH size range. Following chart is actual 25 amp load testing data on 16 different batteries I did. They are all good batteries. These batteries were on float charge for a several months directly before testing so the initial voltage slump in the first minute is the excess lead-oxide build up on positive plates burning off. Once the lead-oxide is thinned out the loaded voltage rises a bit before continuing its normal discharge curve.

After the 30 minute load, load was removed and battery allowed to rest unloaded for 30 minute prior to taking recover voltage level. The 'A' and 'B' group of 4 each, are about three years older batteries than 'C' and 'D' groups which were only about 3 or 4 months old.


One thing you have to be very careful of when paralleling multiple batteries is one battery developing shorted or very leaky cell. Not recognizing a bad battery can drag down the other good batteries, eventually destroying them. One battery with a shorted cell can overheat other five series cells within that battery causing it to overheat during charging. It will put out a lot of nasty gases also.

Awesome, thanks Guys.

That has given me some education on a few points.

My batteries are Supersafe SLA storage backup batteries manufactured in 2018. They haven’t had repeated draw placed on them in that time…apparently. I’ve only just started using them. Some of the batteries read approx 12.5/6 from sitting in storage for some time. They charged up fine though. None of the batteries have read lower than this.

The batteries are in series of four for 48V X 6 for parallel. While charging, some will read 15V + while others reading say about 13.80V. Once charging has stopped, they equalise some. ATM I only have one equaliser fitted to one series as I’m waiting on the others. These four batteries at rest will read 13.25V. All cable lengths are equal and terminals crimped with the right tool. I do prefer to solder but there was the potential to burn the sheath. Cable between batteries, series group and to inverter are 16mm2.

Maybe I need to give the batteries a seperate init charge or will they eventually pull up on their own in series/para?

There is a side by side fridge, 20 y odd fridge/freezer and an old Chevy chest freezer which are the only items being serviced by this battery. Considering having 4080ah i would have thought these would have minimal impact!

Cheers guys.

jberger said:

The batteries are in series of four for 48V X 6 for parallel. While charging, some will read 15V + while others reading say about 13.80V

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This is a catastrophe in process. Critical that you fully charge 12V individually to full before putting them in series. Better if you can repeat the charge with all 4 in parallel.

The 15.0V is being destroyed by overcharging and the 13.8 is being degraded from undercharging.

If individual charging isn't an option, recommend you document your battery voltages at peak system voltage and re-configure the bank with each 4S string of 12V to have as close to the same voltage as possible.

AlanH said:

Once charging has stopped, they equalise some.

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Even ones very out of balance will do so.

AlanH said:

ATM I only have one equaliser fitted to one series as I’m waiting on the others.

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Balancers are great for maintaining balance. They suck for balancing wildly imbalanced batteries. They may say 2A, but that's only when there's a full 1V difference between adjacent batteries.

AlanH said:

Maybe I need to give the batteries a seperate init charge

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Not maybe.

AlanH said:

or will they eventually pull up on their own in series/para?

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Not before damage occurs.

Thanks Sunshine,

Checked voltages this morn while charging by sun and they are all in the low 13’s. For today I’ll monitor them and see how they fair. Oh I didn’t expect the balancer to equalise such diff voltages. These 4 were pretty much even. Oh yeah I know 15v plus was an issue. Yesterday was the first time they got a true solar charge. Today the batteries look to be closer in voltage while charging. All in the low 13’s.

Have these batteries been sitting without being floated or maintained in any way for going on 4+ years?

cs1234 said:

Have these batteries been sitting without being floated or maintained in any way for going on 4+ years?

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No, they came out of service and have been sitting for 6 months or less.

The sun/solar is now charging through an Epever 50A controller. The first battery of every string is now getting up to 15V while the others are in the 13’s. So the first battery is taking the charge then dispersing it to the others. Would the 15V indicate a charge perameter is too high?

Epever 5415 AN

Considering 4 batteries in series, is the controller taking an average of the 4 batteries instead of one battery being to high? It would have to be. They aren’t that smart I suppose. I’ll drop the charging limit.

AlanH said:

No, they came out of service and have been sitting for 6 months or less.

The sun/solar is now charging through an Epever 50A controller. The first battery of every string is now getting up to 15V while the others are in the 13’s. So the first battery is taking the charge then dispersing it to the others. Would the 15V indicate a charge perameter is too high?

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6 months is a long time for lead acids in various states of charge to sit for 6 months not being floated.

I agree with the others that some of these batteries are unhappy and need to be checked and charged individually using 12v chargers. Some probably need desulfacating, water added, and a nice long deep charge.

You can't run these batteries in a 4s6p setup right now. They are all individuals and needed to be treated with kids gloves until their unique needs are met.

Also, how much solar is actually getting to them each day using that controller.. that's a lot of lead acid to charge.

cs1234 said:

6 months is a long time for lead acids in various states of charge to sit for 6 months not being floated.

I agree with the others that some of these batteries are unhappy and need to be checked and charged individually using 12v chargers. Some probably need desulfacating, water added, and a nice long deep charge.

You can't run these batteries in a 4s6p setup right now. They are all individuals and needed to be treated with kids gloves until their unique needs are met.

Also, how much solar is actually getting to them each day using that controller.. that's a lot of lead acid to charge.

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They are sealed so no water can be added. Sealed Lead Acid, SLA.

They are currently getting 22A.

Some details on the battery

AlanH said:

They are sealed so no water can be added. Sealed Lead Acid, SLA.

They are currently getting 22A.

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I'm terrible with math, somebody else here can probably chime in with whether that is enough wattage to charge this bank back up from what was probably empty or not.

Sorry I missed the SLA part, that isn't necessarily better in this situation, some of them may have already vented a bit from overcharging and they can't have water added during their recharging / reconditioning process.

You would probably have a stronger setup for your inverter right now if you just took the best four or eight of them (match by voltage while resting for now) and stuck those together and then tried to recharge the rest individually. Once you get another set of four or eight going, do the same for the set that was running your stuff. Once they are all running great, stick em back into your 4s6p pack and enjoy.. I don't think you will 24 survivors though.

None of the batteries are doing each other any favors while working together mixed and matched right now.

AlanH said:

there is 4080 ah of storage?

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Just noticed this bit now that we're talking 48V. You only have 4S6P; 6 * 170Ah = 1,020Ah of storage.

cs1234 said:

6 months is a long time for lead acids in various states of charge to sit for 6 months not being floated.

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VRLA (SLA/AGM/GEL) tend to lose < 3%/month. The 12.5-12.6V measured when taken out of storage suggests they were at >80% charge after 6 months. Not ideal, but not terrible.

AlanH said:

They are sealed so no water can be added. Sealed Lead Acid, SLA.

They are currently getting 22A.

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It depends on the manufacturer, but generally, 0.06 - 0.15C is a safe range. 170Ah * 6 = 1020Ah

Based on the provided battery cyclic specification, they want 0.1C, so you should be charging the bank at a MINMUM 102A.

There is some nuance in the above. When you're in absorption mode, you're voltage limited, so it will naturally draw less amps than the above., but when below 14.4V/battery, they should be fed 102A+

AlanH said:

Considering 4 batteries in series, is the controller taking an average of the 4 batteries instead of one battery being to high? It would have to be. They aren’t that smart I suppose. I’ll drop the charging limit.

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Good idea. Charger is as smart as it needs to be. The installation is the issue.

Running these SLA to anything above 14.4V, let alone 15.0V, is potentially rapidly shortening their lives - even more likely given that they're old.

Recommend you sort the batteries as I prescribed and match within a string. For any outliers, simply take the whole string out of the bank and deal with them individually.

You need to charge individual 12V at a minmum of 17A to 14.4V and hold until the current drops to 3.4A.

Summary of issues contributing to reduced battery life:

1. Imbalanced string components causing over-charge/under-charge.
2. Insufficient charge current

Given that these have been in storage, recommend you conduct a "periodic boost charge" per 5.4 once you have sorted the string issues.

I assume you got these used and cheap. That's good. These likely aren't going to last that long given their age and nature (seem intended more for UPS-type operations). Even brand new, cycle life of VRLA batteries is typically not much more than 3 years (degradation from 100% to 80% rated).

@sunshine_eggo

Despite your reputation for insulting people, you are truly a valuable asset to this forum. I can't believe you post so much, so quickly, and the majority of it good stuff.

I tip my hat to you sir!

cs1234 said:

@sunshine_eggo

Despite your reputation for insulting people, you are truly a valuable asset to this forum. I can't believe you post so much, so quickly, and the majority of it good stuff.

I tip my hat to you sir!

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Given enough time it will probably happen.

I tend to be blunt as shit, and it doesn't always go over well.

Majority? You're being generous.

sunshine_eggo said:

Just noticed this bit now that we're talking 48V. You only have 4S6P; 6 * 170Ah = 1,020Ah of storage.

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Only, LOL. He's trying to run two fridges and a freezer. He needs to stop opening and closing them so much. Only around here would we describe 48v * 1020ah as "only".