Two AGM in Parallel. One gets warm.

[s/v Jedi]

Originally Posted by allendick

I doubt he knows. I have a Heart monitor and a Balmar Smartgauge and they appeared to have the roughly full rated capacity until recently according to both. They did get discharged to 30 % at one point and in the last while the monitors have not agreed. Coincidence?

I don’t know, I have no experience with those monitors.

[allendick]

Thanks for the help. I am going to have to work this out over the next few days. What I see is the opposite of what I expected. The warm battery is holding the voltage. The cool one, not so much.

Right now the warm one is disconnected and I'm running on the charger and the cool one (which dropped to 12.66 before I turned on the switch and charger). The other is stil at 12.81.

I'll resume the project tomorrow. I may equalise. We'll see. I need to think about it.

Just the same, I'll go to lithium before too long.

[s/v Jedi]

Originally Posted by allendick

Thanks for the help. I am going to have to work this out over the next few days. What I see is the opposite of what I expected. The warm battery is holding the voltage. The cool one, not so much.

Right now the warm one is disconnected and I'm running on the charger and the cool one (which dropped to 12.66 before I turned on the switch and charger). The other is stil at 12.81.

I'll resume the project tomorrow. I may equalise. We'll see. I need to think about it.

Just the same, I'll go to lithium before too long.

I believe Lifeline has instructions on equalizing them. It sounds like the one that went down to 12.66V may need that, but you can also make it work with a couple deep cycles, like down to just 20% state of charge, then fully recharge again. Often batteries regain most of lost capacity again from that.

[witzgall]

Do you have the battery cables attached to only one battery, or is the positive on one and the negative on the other? This can lead to unequal charging/discharging if they are both on the same battery. While you are at it, check the condition of the cables very carefully.

Chris

[allendick]

They are set up properly. The negative is on one, the positive on the other.

[donradcliffe]

My first thought is that you may have a bad connection to the hot battery, and resistance heating around the bad connection. Clean the cables and posts and try again.

If that doesn't solve the problem, unplug the charger and solar, hook up one of the batteries, and do a 20 hour load test. That means turning on enough stuff to pull 10 amps and see how long it takes for the 200 ahr battery to get down to 10.5 volts. If it lasts more than 10 hours try the same test on the other battery.

[mako]

Question: shorted cells on old FLA batteries are a cause of fires. Does that same hazard exist with AGMs?

[allendick]

All connections are perfect. My thoughts mirror yours. I just have to figure out how to load test.

First, I need an appropriate load that can be attached and then I need to monitor voltage.

Second, I have to be here, checking regularly, and 20 hours is longer than my waking day.

I could do a 10-hour or even 6-hour, and of course it is just a guess how long it will take at a given load if the batteries have lost capacity.

Right now I am running on just the cool battery.

I just checked the one that was warm and is now disconnected. Even 12 hours later the voltage is 12.77. It was 12.82 when I disconnected it. (It had been on float).

I am now suspecting that this is the good battery and the other might be high resistance.

[s/v Jedi]

Originally Posted by mako

Question: shorted cells on old FLA batteries are a cause of fires. Does that same hazard exist with AGMs?

Yes it does.

Originally Posted by allendick

All connections are perfect. My thoughts mirror yours. I just have to figure out how to load test.

First, I need an appropriate load that can be attached and then I need to monitor voltage.

Second, I have to be here, checking regularly, and 20 hours is longer than my waking day.

I could do a 10-hour or even 6-hour, and of course it is just a guess how long it will take at a given load if the batteries have lost capacity.

Right now I am running on just the cool battery.

I just checked the one that was warm and is now disconnected. Even 12 hours later the voltage is 12.77. It was 12.82 when I disconnected it. (It had been on float).

I am now suspecting that this is the good battery and the other might be high resistance.

Lifeline also has a 10hr capacity in their specifications so you can simply do a 10hr test instead of the 20hr test using the higher load from the specs.

[allendick]

Fire was my big concern, although there does not seem to be a short. High resistance in one is more likely I think.

I suspect that the ten hour test won't take anything like ten hours on four+ year old batteries.

I just need to decide on a load and set it up, while still having 12V power for my appliances, inclusing this Internet (Starlink).

[s/v Jedi]

Originally Posted by allendick

Fire was my big concern, although there does not seem to be a short. High resistance in one is more likely I think.

I suspect that the ten hour test won't take anything like ten hours on four+ year old batteries.

I just need to decide on a load and set it up, while still having 12V power for my appliances, inclusing this Internet (Starlink).

Simply use one battery to run the boat while testing the other one. Maybe you have some DC powered appliance for the load or a separate inverter?

[allendick]

That is the plan, but I do not have boosters so the cabling and layout is the issue.

I suppose I could just turn things on, turn the chargers off, use the boat circuit as load and record current draw and voltage periodically for a quick and dirty estimate.

No wiring needed. I just have to find the ten-hour test draw for the GPL-8Ds.

Then switch batteries.

[allendick]

I put a 15-Amp load on the cool battery and turned off the charger.

Immediately the voltage was lower than I expected under such a light load. At one point I tried to run the microwave oven, not ever an issue with both connected, even at 50% battery.

11:45 15A 12.06 V
12:10 13A 12.11 V - Could not power the microwave.
12:45 15A 11.98

I'm guessing that this is the problem battery,

[Cheechako]

Originally Posted by allendick

I put a 15-Amp load on the cool battery and turned off the charger.

Immediately the voltage was lower than I expected under such a light load. At one point I tried to run the microwave oven, not ever an issue with both connected, even at 50% battery.

11:45 15A 12.06 V
12:10 13A 12.11 V - Could not power the microwave.
12:45 15A 11.98

I'm guessing that this is the problem battery,

Just charge both, immediately disconnect and let them sit. Measure voltage in a few hours. The one that stays charged is the good one.

[Pete7]

Originally Posted by mako

Question: shorted cells on old FLA batteries are a cause of fires. Does that same hazard exist with AGMs?

Oh yes very much so. I look after a server at work that has a UPS with 12v AGM batteries, eight I think. This is the second UPS, the alarm when off on the first one so went to investigate. The smell was the hint that all was not well. The scorching hot metal case also a bit of a worry. Disconnected and using a truck trolley wheeled it out into the car park. Still warm the following morning from the boiling acid inside. We didn't even bother to open it up, just scrapped the whole unit. We were lucky because it was in the same rack as the server and we were in work. If it had been a weekend the ending could have been quite different.

The problem is normally a shorted cell so the charger see 10v and thinks the battery needs charging so pumps power into it. This turns into heat and just builds up and up.

Pete