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AGM battery thermal runaway

 
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pcg



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PostPosted: Sun Dec 16, 2018 10:24 am    Post subject: AGM battery thermal runaway Reply with quote

I have a question about using a charger that can supply more than 15A to an AGM battery. I’m designing the electrical system for my project boat and want to resolve a concern. Here’s the scoop…

I have a small camper for my truck. I normally keep it plugged into “shore power” while being stored off the truck. This fall I discovered that it had been unplugged for almost six months and the nine year old AGM house battery (Powersonic PS-121000U 12 Volt, 100 AH) was dead. I plugged the camper back in and walked away. A couple days later I opened the camper and was greeted by the smell of hydrogen sulfide. I immediately unplugged the camper and left the door open to let things air out. An hour later I went in and examined the battery, which was still almost too hot to touch. It had swelled and the case was cracked at the top, allowing gases to escape.

In summary the battery died due to thermal runaway. It was charged too rapidly and became overheated. The Powersonic tech told me that, in general, their AGM batteries should never see more than a 10A charging rate, although up to 15A was OK for very short periods. When asked why I had no problem until last fall, given that the camper had a Magnetek 45A charger, he said that because the battery was never discharged very far, the voltage differential between the battery and what the charger was presenting, combined with the relatively high internal resistance of a healthy battery, limited the rate at which the charger was charging.

However, given that I had allowed the battery to reach a very low voltage, the charger started dumping a lot of current into the battery, which raised the temperature of the battery, which lowered its internal resistance, which allowed even more current to flow, which raised the temperature of the battery even more, which allowed even more current to flow… until the battery became so hot that it swelled up and cracked its housing. Hence the term, thermal runaway.

Given what the Powersonic tech told me, I replaced my camper charger with a three-stage 15A charger (Powermax PM3-15LK) to prevent this from happening again, even if the AGM battery becomes completely discharged again. Given that I got good life out of the previous battery, I replaced it with the same one.

Now I’m looking at batteries and a smart charger for the boat, and am reading that people are using three stage chargers rated at more than 15A for stage one, even with an AGM battery. Can these chargers potentially cause an AGM battery to go into thermal runaway if the battery is allowed to reach a very low charge?

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DavidM



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PostPosted: Sun Dec 16, 2018 11:17 am    Post subject: Re: AGM battery thermal runaway Reply with quote

pcg wrote:
In summary the battery died due to thermal runaway.


No, no, no!!! The battery died because it was left discharged for months. If it were a standard flooded lead acid battery, sulfate compounds would have sloughed off of the plates and shorted them causing a low impedence situation that maxes out the charger output when connected.

Something similar happens with AGMs. When you hook up a charger to such a compromised battery, yes it pulls a lot of current and gets hot. That caused the case to crack and truly kill it. But it was essentially dead before.

I strongly disagree with your tech's recommendation to use a charger with an amp rating of less than 10% of the batteries amp hour capacity. Yes, that could limit the high current that you experienced, but it wouldn't change the fact that the battery was badly compromised by letting it run down.

FWIW, most manufacturer's say that both AGM and FLA batteries can be charged at up to 25% of their amp hour rating and some say as high as 50% for AGMs, a far cry from 10%.

The moral is, don't let batteries stay discharged for long periods of time.

David
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thataway



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PostPosted: Sun Dec 16, 2018 5:27 pm    Post subject: Reply with quote

Agree with David--also the other factor is that most likely you had a "converter" which is not really regulated, and made to be used as a power supply in the camper. Because the battery had an internal short--the converter continued with an unregulated charge--and the overheat etc. Most battery chargers would not even charge a shorted battery--the converter of an RV cannot differentiate.
I run 30 amps into a group 31 AGM battery rated at 110 amp hours -regularly. AGM will accept a charger slightly faster than the flooded lead acid.

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pcg



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PostPosted: Sun Dec 16, 2018 5:58 pm    Post subject: Reply with quote

thataway wrote:
most likely you had a "converter" which is not really regulated... most battery chargers would not even charge a shorted battery--the converter of an RV cannot differentiate.
You are correct. It was a Magnetek "Converter Charger".

Thank you both for clarifying what happened and why.
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thataway



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PostPosted: Mon Dec 17, 2018 8:29 pm    Post subject: Reply with quote

The manual for your 15A charger (Powermax PM3-15LK states "never leave the converter charger plugged in when you are not in the RV".

The far better course for you, is to fully charge the battery, with a good quality charger (only--not converter), and then disconnect the battery until you need to use the RV. At that point plug in a charger a day before use, and be sure the battery is up to full power. An AGM battery has a very low "self discharge" rate. As long as all parasitic draws are removed it can be left for 6 months with no charger. The problem is that most RV's and boats have some parasitic draws--like the memory in a stereo, VHF radio etc. There are a few battery chargers which are specifically designed to be left plugged in--and will be "off" most of the time--and every day "poll" the battery condition and apply a small charge if needed. (it may be once a week etc--depending on the circumstance.)

The other option is to remove the battery, or disconnect the negative battery cable. In my RV, I have AGM batteries, and I disconnect them unless I am using the RV. My boat has one of the more sophisticated chargers which samples and apples if necessary.
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pcg



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PostPosted: Tue Dec 18, 2018 7:47 am    Post subject: Reply with quote

thataway wrote:
The manual for your 15A charger (Powermax PM3-15LK states "never leave the converter charger plugged in when you are not in the RV".

You are a careful reader!

I called the manufacturer about that, since it is a three-stage charger and I wanted a charger with a float mode to maintain the AGM battery. He told me it is designed to be left on, unattended during storage, as long as you do not change the factory default settings, and that the default settings are appropriate for being left on unattended, even with an AGM battery.

His explanation for the statement you found was, "That's just sort of a generic statement that applies to other products, that got left in there." !!???
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pcg



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PostPosted: Tue Dec 18, 2018 7:49 am    Post subject: Reply with quote

thataway wrote:
The manual for your 15A charger (Powermax PM3-15LK states "never leave the converter charger plugged in when you are not in the RV".

You are a careful reader!

I called the manufacturer about that, since it is a three-stage charger with a float mode. The tech guy told me it is designed to be left on, unattended during storage, as long as you do not change the factory default settings, and that the default settings are appropriate for it being left on unattended, when connected to an AGM battery.

His explanation for the statement you found was, "That's just sort of a generic statement that applies to other products, that got left in there." !!??? Strange to me because it explicitly calls out that line of charge converters.

I have no parasitic devices in my camper so I may try unplugging it for awhile and monitor the battery voltage from time to time. I have an analog voltmeter wired into the system, but I also have a switch to take it out for that reason.
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BTDT



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PostPosted: Thu Dec 20, 2018 3:26 pm    Post subject: Reply with quote

pcg wrote:
thataway wrote:
The manual for your 15A charger (Powermax PM3-15LK states "never leave the converter charger plugged in when you are not in the RV".


I called the manufacturer about that, since it is a three-stage charger with a float mode. The tech guy told me it is designed to be left on, unattended during storage, as long as you do not change the factory default settings, and that the default settings are appropriate for it being left on unattended, when connected to an AGM battery.

His explanation for the statement you found was, "That's just sort of a generic statement that applies to other products, that got left in there." !!??? Strange to me because it explicitly calls out that line of charge converters.
.


I would not use a charger that specifically states in the owner's manual that you must be present at all times in order to use it. Are you suppose to stay awake at all times too? I would at least ask the manufacturer to put it in writing that it is safe to use the charger unattended.

I also would not purchase a battery whose manufacturer limits the charge to 10% when many outboard motors have 50 amp+ alternators. A 200 amp battery would be limited to a 20 amp charge rate.

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