Battery Fires

[lenny]

Dog killed as e-bike fire rips through home and 'reduces everything to ashes'

Fire crews spent more than six hours at the scene of the devastating blaze in Newton Heath

www.manchestereveningnews.co.uk

 

[chris_n]

Overcharging ought not to be an issue, a full function BMS protects against it whether due to over-voltage, over-current or over-temperature.

Sadly your advice is still good advice because of the under- and un-protected batteries both in circulation and still available. But it's worth reminding ourselves because 'the wrong charger' is a complete red herring and not the path to take going forward concerning deficient batteries.

Can't say I agree with that, a BMS can only dissipate so much excess current when the battery is balancing. If you give it too much current the balancing resistors will surely overheat.
I had a battery overheat as I left it on charge too long, the battery and charger came from a respected German battery builder. The balancing resistors got so hot they were melting the casing! This was a 2A charger on a 400Wh battery

 

[Ghost1951]

Can't say I agree with that, a BMS can only dissipate so much excess current when the battery is balancing. If you give it too much current the balancing resistors will surely overheat.
I had a battery overheat as I left it on charge too long, the battery and charger came from a respected German battery builder. The balancing resistors got so hot they were melting the casing! This was a 2A charger on a 400Wh battery

That has to be about poor design. The resistance of the load balancing resistors needs to be chosen carefully as does the current handling capacity of that system. Heat sinking would be needed if the balancing current was high enough to cause the kind of problem you describe.

I am sure that part of the problem of battery fires is the drive to make batteries and their protection systems as cheap as possible. Another part is the hysterical hyperbolic publicity given to a small number of fires. Information from the big insurers shows that the numbers of fire failures in batteries is tiny by comparison to other causes of fire. I posted information from an insurance company the other day about air fryer fires which are happening in very large numbers and causing an average of £30,000 in damage per episode. There is hardly any press on that. In the lists fire causes provided by insurance companies that I have seen , e-bike battery fires don't even register, but they show tens of thousands of cooking appliance fires and other causes - again - no hysterical press about those.

This table is from Morgan Clark UK Loss Adjusters.

What are the biggest causes of house fires?
By far, the biggest cause of house fires in the UK is cooking appliances, accounting for 44% of all fires.

Cause of fire	Incidents	% of total
Cooking appliances	10,176	44%
Electrical distribution	3,148	14%
Other electrical appliances	2,658	12%
Smoking-related materials	1,522	7%
Matches & candles	926	4%
Space heating appliances	761	3%
Central and water heating appliances	177	1%
Cigarette lighters	149	1%
Matches	117	1%
Blowlamps, welding and cutting equipment	106	0%
Unspecified	3,268	14%

Causes of house fires in England in 2023/24


Source: https://www.morganclark.co.uk/about-us/blog/uk-house-fire-statistics/

 

[saneagle]

Can't say I agree with that, a BMS can only dissipate so much excess current when the battery is balancing. If you give it too much current the balancing resistors will surely overheat.
I had a battery overheat as I left it on charge too long, the battery and charger came from a respected German battery builder. The balancing resistors got so hot they were melting the casing! This was a 2A charger on a 400Wh battery

What goes through the bleed resistors depends on the charger's voltage, not the charging current. The bleed resistors are opened when any cell goes above 4.2v, and they provide a path to ground, so the amount of current that goes through them is 4.2 divided by the resistance value, which is typically 100 Ohms. The current is therefore 42 miliamps and the power dissipated is 176 miliwatts for each one that's open. The resistance value doesn't really change, so if yours got too hot, the logical reason is that your cell voltages were too high. That means that either your charger is wrongly set or your battery is wildly out of balance. You should check both and sort out what the problem is.

Another explanation could be that you covered the battery with something that's a good insulator while charging, so the heat couldn't get out.

 

[chris_n]

I'm not going to disagree with either of the responses to my post.
What I will say is that @saneagle response is exactly why the problem exists. If you use a 48V charger on a 36V battery then you have the situation he describes. The battery wasn't out of balance, nor was it covered by any insulation.
The equipment was as supplied by a reputable supplier so to insinuate that I may have caused this problem is absolute b0ll0x. Even if the battery was wildly out of balance how would the average end user be aware of that?
If the voltage of the as supplied charger is incorrect how would the average end user be aware of that?
Say what you want about closed systems but in my experience they monitor individual cell voltages much better than generic stuff.
As for the statement I should check both and sort them out, that is exactly what I did but expecting the average end user to do that is utter stupidity.

 

[saneagle]

I'm not going to disagree with either of the responses to my post.
What I will say is that @saneagle response is exactly why the problem exists. If you use a 48V charger on a 36V battery then you have the situation he describes. The battery wasn't out of balance, nor was it covered by any insulation.
The equipment was as supplied by a reputable supplier so to insinuate that I may have caused this problem is absolute b0ll0x. Even if the battery was wildly out of balance how would the average end user be aware of that?
If the voltage of the as supplied charger is incorrect how would the average end user be aware of that?
Say what you want about closed systems but in my experience they monitor individual cell voltages much better than generic stuff.
As for the statement I should check both and sort them out, that is exactly what I did but expecting the average end user to do that is utter stupidity.

I only gave you the theory so that you can figure out what went wrong. Bleed resistors don't normally overheat, so there must have been something special that happened to yours. What was it?

 

[chris_n]

I only gave you the theory so that you can figure out what went wrong. Bleed resistors don't normally overheat, so there must have been something special that happened to yours. What was it?

I don't need to figure anything out, this happened a couple of years ago. I checked the cell voltages, all were equal, can't remember what the charger voltage was and it was a little high so I turned the trim pot down.
I didn't have any more problems but did keep a check when it was coming up to being fully charged so it never actually had the opportunity to be a problem.
I am merely highlighting that an over voltage charger can cause issues even with a BMS present. Maybe the BMS wasn't the best, I don't know but I do know that day I came close to having a fire using equipment as supplied.

 

[saneagle]

I don't need to figure anything out, this happened a couple of years ago. I checked the cell voltages, all were equal, can't remember what the charger voltage was and it was a little high so I turned the trim pot down.
I didn't have any more problems but did keep a check when it was coming up to being fully charged so it never actually had the opportunity to be a problem.
I am merely highlighting that an over voltage charger can cause issues even with a BMS present. Maybe the BMS wasn't the best, I don't know but I do know that day I came close to having a fire using equipment as supplied.

The charger couldn't have caused that if the cells were in balance, since the BMS would shut off charging at around 42.5v, so instead of 1.76 watts of heat, you'd have got 2.02W. Even if you had used a 48v charger by mistake and your BMS failed to shut off, you'd only get 3.26W. That's not enough to melt plastic.