When do you stop charging a deep-cycle AGM battery?

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I just bought my first deep-cycle battery. Spec page is here and datasheet is attached. It will mainly be sitting open-circuit in an office, until there's a power cut, whereupon it will be plugged into an inverter and used to power some electronics; then it will get recharged when the power comes back on.

My first question is, since they provide a voltage range for charging (14.6-14.8V), should I go for the top (14.8) or bottom (14.6) of that range? What if I use the more conventional 14.4V? Or does it not matter? My charger is fully programmable so I can set any absorption voltage needed, I just need tips on maximising performance and life. I'm familiar with battery university BU-403, but that doesn't address this question.

Second, and more importantly, what should be the condition for terminating charge? It's not listed anywhere in the datasheet as far as I can tell. Some resources say to keep charging at absorption voltage (14.xx) until current decays to 3-5% of C20 (which for this battery would be 3% x 36Ah = 1.08 Amps). Again, my charger is fully programmable so I can set any termination current for end of charge (I won't be using float charge, I will simply stop charging at that point and put battery in storage) - I just need some advice on what to set.

So far, I've been using 14.4V as Absorption voltage, 10A as initial/max current (for bulk phase) and 1A as termination current. When I do that, by the end of charging I can hear a very very faint sound coming from the battery when I press my ear right up against it - the sound a flooded battery makes when it's bubbling, only with this sealed battery the bubble-popping noise only happens once every couple seconds. The battery doesn't warm up at all at any point of charging, and the terminal voltage is 13.1V the next day after removing surface charge with a light load. The absorption phase takes about 2 hours.
 

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Ian0

Joined Aug 7, 2020
748
Charge to 2450mV per cell, and stop charging when the current stops falling. When dI/dt reaches zero the battery is fully charged.
If you stop when the current falls to a certain (arbitrary) amount, say C/40, then, as the battery ages it may never fall that low. If that happens your charger stays in absorption until it ruins the battery.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
Charge to 2450mV per cell, and stop charging when the current stops falling. When dI/dt reaches zero the battery is fully charged.
If you stop when the current falls to a certain (arbitrary) amount, say C/40, then, as the battery ages it may never fall that low. If that happens your charger stays in absorption until it ruins the battery.
Fair enough, unfortunately my charger doesn't support dI/dt (it actually stops when current falls below 10% of bulk current). I can however implement your suggested algorithm by observing the charger display and manually stopping when current doesn't change for a while. How long should I wait before considering dI/dt to have reached zero? Say, 30 minutes of seeing the same current reading? The digital current gauge on the charger is accurate to 0.1A.

So far current has always continued to decline slowly the longer I leave it on charge, but after 2h at 14.4V it seemed overkill to keep waiting for the current to decline further. I'd appreciate your thoughts on this.
 

boostbuck

Joined Oct 5, 2017
15
I suggest you invest in a modern multistage charger. This will take the battery through a bulk charge and an absorb stage, and at completion will hold the battery at float voltage, which is the optimum for long term storage.

You shouldn't take an AGM battery to the point where you can hear sounds issuing from it. I would suggest you err on the side of caution (ie lower final charge voltage) to avoid gassing.

The data sheet provides the manufacturer's recommended charging parameters, and your questions are answered by the graphs.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I suggest you invest in a modern multistage charger. This will take the battery through a bulk charge and an absorb stage, and at completion will hold the battery at float voltage, which is the optimum for long term storage.

You shouldn't take an AGM battery to the point where you can hear sounds issuing from it. I would suggest you err on the side of caution (ie lower final charge voltage) to avoid gassing.

The data sheet provides the manufacturer's recommended charging parameters, and your questions are answered by the graphs.
The graph in the datasheet is for standby use (charge at 2.275V/cell) - my application requires cyclic use.

I know sounds from a sealed battery are probably bad because they represent consumption of non-replaceable electrolyte, but they're very, very faint. I had to turn off everything in the house and press my ear right up against the battery to hear it. I was actually deliberately looking for it because I was curious whether sealed batteries would sound like flooded batteries at the end of charge. And the current and voltage were well within manufacturer's specs.

I actually got a reply from the manufacturer this morning by email. Here's what they said:

It is very important to charge the battery with the 14,8 Volt range. If you do 14,4 Volt the battery will not charge fully. And the charger must have a current of 9 Amps maximum.

The best termination is 0,02C ( 0,72 Amp) to turn over to the floatcharge of 13,4 – 13,6 Volt.
I hope you informed well.
 

Ian0

Joined Aug 7, 2020
748
I wait until it hasn't changed for 4 minutes, but my current reading is a bit more precise.
Using a fixed threshold current is fine for a new battery. My method does keep a new battery at a higher voltage for longer, but the current is low. As the battery ages, there comes a time when it will never fall as low as C/50, and the charger will keep it at the higher voltage for ever with quite a lot of current flowing. Probably that is time to replace the battery, but if the charger fails to switch to float, it will ensure that you have to replace the battery!
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I wait until it hasn't changed for 4 minutes, but my current reading is a bit more precise.
Using a fixed threshold current is fine for a new battery. My method does keep a new battery at a higher voltage for longer, but the current is low. As the battery ages, there comes a time when it will never fall as low as C/50, and the charger will keep it at the higher voltage for ever with quite a lot of current flowing. Probably that is time to replace the battery, but if the charger fails to switch to float, it will ensure that you have to replace the battery!
I know what you mean, I have a few old car batteries that will draw 1-1.5A at 14.4V all day, they just bubble gently but the current never decays past that. I just set the threshhold higher, say 1.6-2A so the charger still stops automatically. I haven't left them unattended for long enough to actually destroy them, could you describe what the failure mode from overcharge would look like? Does the cell go open-circuit or develop high internal resistance? I assume a spectacular disassembly event with smoke and flames is unlikely since my charger will never exceed the preset voltage and current limits (14.4-14.7V and 7-15A or so).
 

Ian0

Joined Aug 7, 2020
748
Plate corrosion is the usual mechanism. In a flooded cell, it is generally because no-one remembers to top up the water. It's an awful lot safer than overcharging a lithium cell!
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
Just did a full charge to manufacturer's recommended specs yesterday (14.8V, but temperature-compensated down to 14.6V since battery temperature was 33°C; held until current fell to 2% of C20, which is 0.72A). The complete charge, from 50% DoD, took 4.5 hours. By the end of it the gassing/hissing noise was quite evident.

This doesn't seem right to me. Could it be that the manufacturer's advice is not the best for longevity? The battery seems to gas when the voltage goes above 14.3V. Maybe I should just charge at a lower voltage for a longer time?
 

Ian0

Joined Aug 7, 2020
748
It should start to gas at 14.4V, that's the dissociation voltage of water. That's correct, but recombination should take care of it provided that the charge current is below C/5.
How many AH is the battery, and what was your charging current?
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
It should start to gas at 14.4V, that's the dissociation voltage of water. That's correct, but recombination should take care of it provided that the charge current is below C/5.
How many AH is the battery, and what was your charging current?
Battery is 36Ah. Charge current starts out at 7.2A (0.2C) and is under 5A most of the absorb. The battery label says max charge 10.8A.
 

Ian0

Joined Aug 7, 2020
748
The implication is that it should fully recombine at 10.8A and therefore not gas. I wonder if the vent opens at too low a pressure.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
The implication is that it should fully recombine at 10.8A and therefore not gas. I wonder if the vent opens at too low a pressure.
It starts out with a bubbling noise (like the noise a glass of sparkling water makes) but yesterday it progressed to a light whoosh noise by the end of absorption. I tried blocking the vents with my fingers and didn't feel any gas pressure but I think the gas can escape along the whole seam in the plastic rectangle covering the top caps. Maybe I should put a strip of masking tape to cover the whole seam on the next charge and see if gas pressure pops it off?

I did a small experiment, I first charged at 14.4V down to 0.72A. Then set the charger to 14.6V and did another absorb, and it only took an additional 0.6Ah to terminate charge to 0.72A at the new higher voltage. So the difference in coulombs put in is minor.

Question - should/could I charge at 14.4V to minimise gassing? Or will this kill the battery with sulphation?
 

Ian0

Joined Aug 7, 2020
748
I think that the battery is faulty. A VRLA battery needs to develop some pressure for the recombination process to occur, that's why it's "Valve Regulated". If the gas is escaping, then it's behaving just like a flooded battery, except that you are unable to top it up, so after a few charge cycles it will have dried out. Better have the supplier replace it whilst it's still under guarantee, before they can think up an excuse not to.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I think that the battery is faulty. A VRLA battery needs to develop some pressure for the recombination process to occur, that's why it's "Valve Regulated". If the gas is escaping, then it's behaving just like a flooded battery, except that you are unable to top it up, so after a few charge cycles it will have dried out. Better have the supplier replace it whilst it's still under guarantee, before they can think up an excuse not to.
That would be a pity, it's only a month old and the discharge and charge performance has been stellar so far, though it does get a little warm at the end of charge (room temp 27C, battery temp 33C). I could probably replenish the water by breaking off the top plate and squirting it in through the holes under the rubber caps (assuming construction is the same as a UPS battery).

I will do some testing with the tape to confirm that gas is indeed escaping.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
You try getting your money back on a battery that's more than a few months old! It just doesn't happen.
Fair enough, I will test and see whether replacement is warranted :)

I'm doing this battery test at the moment:
1. Discharge 30% of the capacity (10.8Ah - measured with an accurate inline coulometer)
2. Tape the vents
3. Recharge at 7.2A, 14.4V, until 0.72A. Record Ah in, inspect tape for gas bubbles
4. Recharge further at 14.7V until 0.72A and record Ah in.

Hopefully this will reveal whether it's gassing and whether sufficient Ah are put back in at lower absorbtion voltages (datasheet says to aim to put in 1.1-1.15x the Ah taken out for full charge).
 
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Thread Starter

LMF5000

Joined Oct 25, 2017
87
Results are in!
Start: fully charged, 13.1V open voltage
Discharged exactly 10.800 Ah (30% of capacity).

Recharge: 2.4V/cell (14.4V), 7.2A (0.72A termination current)
Start: 26.6°C
Bulk (CC->CV switchover point): 7.656 Ah (70%), 1h 03m, 29°C, 7.19A
100% point (10.8Ah put back in): 10.800 Ah (100%), 2h14m, 31.4°C, 0.72A
Absorbtion (End of charge): Happened at 100% point (10.8Ah)!

Further recharge: temp compensated for 31.4°C (14.8V minus 30mV/°C becomes 14.608V, i.e. 2.434V/cell, rounded to 2 decimal places = 2.43V/cell = 14.58V), 7.2A (0.72A termination):
56mAh (0.51%), 0h 4m, 30.8°C

Further recharge to 14.7V, 7.2A (0.72A termination):
22mAh, 0h 1.5m, 30.5°C

So basically the battery would only absorb exactly as many amp-hours as I took out, at any voltage between 14.4 and 14.7V, to an end-amps value of 2% of battery capacity. The masking tape on the vents didn't lift, so I assume it's not actually gassing, just making recombination noises?
 
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