Is it safe to seal batteries in a waterproof plastic food container?

opearn

opearn

100 µW
Joined
Oct 11, 2018
Messages
7
I have 2 36v 7.5 10s3p battery packs that fit nicely in a plastic food storage container, As it would be waterproof I intend to add a cable gland and run the wires out of the container, However i would worry about combustibility of a sealed plastic case. Would it be safe or would it cause a fire/battery explosion?



OLD question:
Hi I have 2 old chargers i got with a broken ebike i am repairing.
My main question is is it safe to use a battery charger that shows slightly higher than 42v? say it reads 42.8, do battery chargers go bad/voltage creep or is this not an issue if my battery packs have a bms?
I've also made up some 4pin XLR to dc connectors (checked output of charger with multimeter & wired according to battery sellers instructions), how would i go about i use my multimeter to double check the polarity is right? can i just measure the voltage of the batteries charge connector with my multimeter to see the polarity?
I often read of people charging their batteries to lower voltages say 4.1v sacrificing a small amount of range for extra charge cycles, would a simple voltage divider circuit work or how could i go about retrofitting one of my chargers or make a voltage reducing adaptor to only charge to a lesser capacity (41v?)?
I can take some photos and write up a little soldering tutorial on how i solder connectors if anyone would like aswell.
Click to expand...
 
It is actually very common for chargers to output a slightly higher voltage than they are rated for. Although 42.8V is a bit higher than what I usually see. Most BMSs are set to cut out at around 4.25V per cell. If the charger voltage is higher than that the BMS will switch the current off and on repeatedly in the CV charing phase. This could cause the charging to progress slower than it should.

You haven't made it clear if the XLR is on the battery or charger side. As long as you haven't connected any XLR pins together you could plug it in to either the battery or charger and check the voltage on the other connector. Then check the voltage on the mating connector and confirm that they are both the same.

A voltage divider is a high impedance source and is not suitable for supplying the current to charge a battery. Inside the charger on the output side you will find a voltage divider (either fixed or adjustable resistors) that feeds into a comparator along with a voltage reference. This is used to control a voltage regulator which will limit the output voltage.

Here is a good explanation of how to adjust the output voltage of many PSUs:
https://hackaday.io/project/3469-modifying-a-notebook-power-supply

If you are charging the cells only to 41V they can become slightly unbalanced which will reduce the effective battery capacity. A high quality battery pack usually stays fairly balanced though. It can be a good idea to leave the battery on a charger at 42V for a while every now and again to balance them.
 
Thanks I checked the polarity of the batteries charge port & it matches the connector I made. I also realized the wiring information was on the charger sticker. (but i think it was still good to double check).
I've opened up one of my chargers (there's 4 screws under the rubber feet) and found a potentiometer with VR written next to it, It had a glob of yellow glue on it to prevent adjustment but i removed the glue, connected up my multimeter to output & turned the pot until i read a voltage of 31.9-41.1v. Just don't be silly & touch the 240v side while it's live if you do this.
I figure it would be possible to desolder the potentiometer (P102 t020 1k ohm) & replace with one that would be accessible from outside the case & also solder a voltmeter to the output to allow for easy charge control & be a little more safe.
 
opearn said:
turned the pot until i read a voltage of 31.9-41.1v. Just don't be silly & touch the 240v side while it's live if you do this.
Click to expand...

If you read the Hackaday link, a safer way to do this is to only connect the battery, and then check the optocoupler with a multimeter. Or sometimes you can just look at the charger LEDs.
 
is 42.5v the correct voltage to charge a 10s pack to? i was under the impression that 4.2v is fully charged?
 
4.2V per cell is generally accepted as the full charge voltage but BMSs are often set a bit higher so they won't cut out unless there's a fault or the wrong charger is used. It also needs to be higher than the balancing voltage (which is usually around 4.2V) and have a bit of a margin.
 
Best to not rely on the usual BMS process to keep your cells balanced, just use it for protection, maybe live monitoring.

Balancing can be done regularly, as a periodic maintenance routine as needed.

Thus no need to let the cells sit for long periods at high SoC.

The key is finding the SoC stop-point you want, balancing range and longevity. There is an infinite number of V / A profiles that will safely get you to that point.

At high C rates, going a bit higher on finish voltage, gets you to a similar SoC stopping at lower voltages when current is low.

Just stopping **at** a given voltage is fine, if you don't mind sacrificing a little range, better for longevity.

Stopping CV charge based on amps accepted gives another few %, depending on C rate.

If the voltage is a bit high, just stop a bit earlier, more so at lower C rates.

 
Most of the basic China plastic 2a chargers have a trim port/potentometer inside so you can adjust the voltage if you have a meter that you know to be quite accurate. I have adjusted mine and also find they are good for storage voltage for 13s, adjust to 50v and you get a tad under 3.85v per cell for 13s.
 
would it be safe to put my lithium battery packs in a sealed plastic container during use discharging on a bike or would it be a fire risk? thinking want the batteries to be waterproof.
 
Yes you can. Just use at below rated level. What are the batteries that you are using.? What is your motor controller setup ? Amp draw is what makes heat.
 
opearn said:
would it be safe to put my lithium battery packs in a sealed plastic container during use discharging on a bike or would it be a fire risk? thinking want the batteries to be waterproof.
Click to expand...
No issues with being sealed. A pretty big issue with heat, though, since heat + high states of charge kill li-ion batteries very quickly.
 
opearn said:
would it be safe to put my lithium battery packs in a sealed plastic container during use discharging on a bike or would it be a fire risk? thinking want the batteries to be waterproof.
Click to expand...

I have a pack that's built into a pair of Pelican cases. Healthy lithium batteries don't give off any gas, so no issue there. If a cell decides to catch on fire from internal short or over charge, then the plastic will melt pretty fast.

I have the bottom of the pack glued into the box with silicone. This helps a lot with heat dissipation.

Img_1121.jpg


Img_1144A.jpg
 
There are also electrically insulating compounds, maybe even adhesives, that are thermally conductive.
 
999zip999 said:
Yes you can. Just use at below rated level. What are the batteries that you are using.? What is your motor controller setup ? Amp draw is what makes heat.
Click to expand...

I am using 2 packs in parralel they are 10s3p 36v 7.5ah & each are rated for 20a constant discharge, 60a instantanious, which should give me 40a constant current. I'm using a 200w controller (draw should be like max 14a as a guess) but may upgrade to a 17A 350w.

Do they vent gas or generate excessive heat while charging? I've heard its best to charge lithium cells in fireproof boxes, is it safe to just remove/open the plastic case & leave in them to charge?


999zip999 said:
What part of Australia the dry side or the Wet Side.
Click to expand...
I'm on the cold little island on the other side of bass straight.
 
They don’t give off any gas during charging like lead-acid cells. The container can stay sealed. I put a small hole in the bottom of my boxes to allow water to drain if it gets in.
 
But charging at higher rates does raise temps and stresses the bank.

If you aren't in a hurry, 0.2-.3C rates are conducive to longevity, even lower if you're willing to leave overnight.

Which of course no one should ever do :cool:
 
Bottom line in my opinion, if heat on charging or discharging is a problem, your problem is not that the battery is in a pelican case, or a Tupperware.

If its that hot, then you have a big safety problem all right, and nothing will make it safe if your charging and discharging is making it too hot to touch.

Even a metal ammo box is not certain to be safe once that battery bursts into flame. A metal box might get you some time to escape, but that is all. It would have to be vented with lots of vent, or it becomes an artillery shell when it catches fire. Metal case batteries are sealed, but in a fire, the ends will pop off when the gas pressure gets too much. I store batteries in ammo cans, but don't latch them down.

but yes, go ahead and use any handy plastic, or metal container to keep your battery dry. Right now on my dirt e bike, I found a plastic ammo box for shotgun shells the perfect fit for 12s 10 ah of lipo. I did not grommet the hole for the wire, but would shove a plastic bag over the whole box if caught in that bad a downpour. Riding it, my ass shades the hole from falling rain.

You are right to worry about safety. I thought I was perfectly safe, till my wife woke me up to get out of the burning house. I charge and store bike batteries outside now. But the particular danger IMO, is when you charge, when the battery gets full and a charger fails to stop, or cells fail before the charger stops normally. This is what happened to me I think, the battery was almost full, when a damaged cell got overcharged. The bms may have failed, allowing this to happen.
 
dogman dan said:
Bottom line in my opinion, if heat on charging or discharging is a problem, your problem is not that the battery is in a pelican case, or a Tupperware.

If its that hot, then you have a big safety problem all right, and nothing will make it safe if your charging and discharging is making it too hot to touch.

Even a metal ammo box is not certain to be safe once that battery bursts into flame. A metal box might get you some time to escape, but that is all. It would have to be vented with lots of vent, or it becomes an artillery shell when it catches fire. Metal case batteries are sealed, but in a fire, the ends will pop off when the gas pressure gets too much. I store batteries in ammo cans, but don't latch them down.

but yes, go ahead and use any handy plastic, or metal container to keep your battery dry. Right now on my dirt e bike, I found a plastic ammo box for shotgun shells the perfect fit for 12s 10 ah of lipo. I did not grommet the hole for the wire, but would shove a plastic bag over the whole box if caught in that bad a downpour. Riding it, my ass shades the hole from falling rain.

You are right to worry about safety. I thought I was perfectly safe, till my wife woke me up to get out of the burning house. I charge and store bike batteries outside now. But the particular danger IMO, is when you charge, when the battery gets full and a charger fails to stop, or cells fail before the charger stops normally. This is what happened to me I think, the battery was almost full, when a damaged cell got overcharged. The bms may have failed, allowing this to happen.
Click to expand...

I read your story about the fire and it does bother me. I am getting an EM3EV HG2 14S5P battery, which should be fairly safe. However, I don't work close to home. I rent a small apartment near where I work and the bike will be stored there. It has a tiny patio (~6'X8'), but nowhere to build an outdoor battery bunker. I bought a metal ammo can to keep the battery in while it is charging and in the apartment and a rolling metal cart to wheel it outside in the event of a fire. I suppose I could store the ammo can on the patio. I was planning to remove the seal on the lid of the ammo can to allow it to vent.
 
The higher the quality of the battery the less likely for any type of fire. Remember there are many many batteries out there on the road. Em3 makes a quality battery. Saying that anything can happen in this world
Correct me if I'm wrong but dog man's battery was a cheap Chinese knock-off of a battery
 
999zip999 said:
The higher the quality of the battery the less likely for any type of fire. Remember there are many many batteries out there on the road. Em3 makes a quality battery. Saying that anything can happen in this world
Correct me if I'm wrong but dog man's battery was a cheap Chinese knock-off of a battery
Click to expand...

Yeah, I understand that. I haven't read of any EM3EV batteries failing in a pyrotechnic sort of way. There aren't very good statistics available on the failure rate of these things.
 
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