Is there a way to do controlled battery discharge?

[E-HP]

(a 15W light bulb's glass surface temperature won't get that high because it's too large an area for the heat generated inside at the filament and the distance of vacuum between them, but a resistor has direct conduction from the heat source to the much smaller surface area, so even a 15w resistor could soften or even melt some plastics if enclosed by them).

Maybe one of these, partially submerged in a bucket of water, or a couple of those coffee cup water warmers to provide something to absorb the calories...

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[raylo32]

Here is the 300w 10 ohm power resistor I got off Amazon. It's a monster with lots of surface area to dissipate heat. It has no mounting posts or tabs so I will need to figure something out, which shouldn't be that difficult. All it has are the two electrical connection terminals. I put a meter on it and it measured 10.2 ohms. I'll first just make up a pigtail and give it a test run to see what kind of surface temps it produces before I decide how to mount it. But this project will be on the back burner for a while.

 

[Comrade]

Here is the 300w 10 ohm power resistor I got

Why 10 ohms? It will dissipate more than 300w on a fully charged 14 cell battery. Are you in a rush to discharge? I would have picked a size that would bring the battery down to say 75% from 100% in a known time period. Like 24 hrs. You plug it in when you do some daily chore one day, then unplug it the next day doing the same daily chore. Easy to not forget.

 

[lnanek]

Most fires occur when either charging or discharging. Even when I use a slow incandescent bulb to discharge a battery down to storage charge state, I unplug that when I sleep or leave the house and resume when I'm back.

That said, if I was going for fast discharge like he did, I would have gotten something with a fan.

 

[raylo32]

Can easily add a fan or put it in front of one if needed.

Most fires occur when either charging or discharging. Even when I use a slow incandescent bulb to discharge a battery down to storage charge state, I unplug that when I sleep or leave the house and resume when I'm back.

That said, if I was going for fast discharge like he did, I would have gotten something with a fan.

 

[Comrade]

Most fires occur when either charging or discharging.

a.) Gently discharging using a resistor?

b.) Or discharging by short circuit or severely overloaded?

 

[raylo32]

Yeah, I probably should have gone with a 12 ohm.... and may well change over to that. As I said this is back burner for now. And I did pick a time. Just a lot shorter than a day. When these situations occur, I don't have a day to wait. And a ~5 amp discharge is really not high for such a battery that has a 50amp BMS.

Why 10 ohms? It will dissipate more than 300w on a fully charged 14 cell battery. Are you in a rush to discharge? I would have picked a size that would bring the battery down to say 75% from 100% in a known time period. Like 24 hrs. You plug it in when you do some daily chore one day, then unplug it the next day doing the same daily chore. Easy to not forget.

 

[E-HP]

When these situations occur, I don't have a day to wait. And a ~5 amp discharge is really not high for such a battery that has a 50amp BMS.

If you have a hill nearby, the fastest and safest way to discharge is to ride the bike for 15 minutes. This situation always manages to occur on the rare occasion where I fully charge my pack before going on a long ride, and my wife says something like "hey let's watch the F1 race, it just finished recording" to which I reply, "oh ya, get it set up and give me 15 minutes, I just charged my battery", at which point I do a couple of high speed runs to the top of the hill and back. That's enough to pull it down a couple of volts out of the danger zone. 10 minutes at 30A is 5Ah.

 

[eMark]

Assuming your 14s battery is AOK then storing it for two weeks at 90% shouldn't deplete cycle life (others may disagree preferring 80% storage for a 2 week period (58.5v - 3.0v = 55.5v = 80% usable 14s energy for prolonging cycle life).

58.5v - 43..4v (cut-off) = 15.1v .... 10% of 15.1v = 1.5v ... 58.5v - 1.5v = 57v (resting volts)

Remember that there is bounce back voltage after a power/speead ride resulting in at least another volt after a 2 hr rest. So drain pack voltage to say 55v. After 2 weeks in storage the resting 14s pack voltage should be around 57V. With a good battery pack there shouldn't be any significant self-discharge after storing for two weeks. Best to record voltage when first put in storage and again after a couple weeks.

Note: Sorry for error ... refigured at 43.4v (cut-off) ... was mistakenly thinking 31.5v cut-off (my 10s battery)

 

[harrisonpatm]

I think it'll be a fine setup. I use the same method to discharge 40ea parallel 18650 cells at a time, during my recycling/sorting/testing/storing, it works great. I have no fancy electronics or anything, just a 40-cell rack with an xt60 connection, and the other xt60 to a resistor bank. Just watch the voltage, set a timer, ect...

 

[harrisonpatm]

It has no mounting posts or tabs so I will need to figure something out, which shouldn't be that difficult. All it has are the two electrical connection terminals.

Metal conduit mounting clamps on a piece of wood.

 

[amberwolf]

Those are usually hollow. You can use a piece of copper tubing (or any metal pipe, but the copper will help pull heat out) with the same OD as the ID of the resistor, and flare one end of the tube out enough to have a flattened end to screw down to something, or bend the tube into a squared-U shape like a toiletpaper roll holder.

 

[Diggs]

All this discussion and no one uses a battery discharge tool like the DL24? Any load (0-185 watts) and any voltage (0-200 volts) can be set. It tracks and monitors Wh, mAh, time, temps, etc. and costs as little as $25. I'm surprised this isn't in common use considering $25 is cheap to check a $350-$500 packs. My first use is to cycle new battery packs so I know how much capacity is in them so I have a baseline and can check how they are holding up over the years.

 

[docw009]

All this discussion and no one uses a battery discharge tool like the DL24?

I use a DL24 to qualify my battery packs. I also use it to set the battery at 3.3V/cell storage level. It's a little fragile. For folks that have 6p packs or higher, Its limitation is the 180W max.

 

[harrisonpatm]

All this discussion and no one uses a battery discharge tool like the DL24? Any load (0-185 watts) and any voltage (0-200 volts) can be set. It tracks and monitors Wh, mAh, time, temps, etc. and costs as little as $25. I'm surprised this isn't in common use considering $25 is cheap to check a $350-$500 packs. My first use is to cycle new battery packs so I know how much capacity is in them so I have a baseline and can check how they are holding up over the years.

View attachment 340037

I use that too, for testing. And it's great, and the plus is that you can set the low voltage cutoff, meaning that you can discharge your battery to a set voltage, and when it reaches that voltage it'll stop discharging. Essentially, you can set it to discharge your battery, then walk away, knowing it'll stop discharging without killing the battery (note: don't actually do this, lithium should always have some level of attention).

The primary reason I didn't originally suggest it is because it requires a separate 12v power source to run. Obviously it doesn't need much current or power at 12v to discharge the battery, it's basically just running that LCD. But it does mean OP would need to plug in the board, and then plug in the battery. Whereas a resistor just drains the battery on its own.

Still not a bad option though. Maybe the separate 12v supply isn't an obstacle. And it certainly can also be multitasked as part of one's battery testing equipment.

 

[Diggs]

it requires a separate 12v power source to run

Just a note - It is not polarity protected from a 12 volt source. I run an old 12 volt computer supply for 12 volt supply (16 amps on the 12 volt rail) on the bench. I wired in a barrel connector for the DL24 and didn't realize the connector I grabbed was reverse polarity with negative in the middle and positive on the outside barrel. I got one small spark from that DL24 when I plugged it in and it was all over. Fried it! Be aware!

 

[Diggs]

For folks that have 6p packs or higher, Its limitation is the 180W max.

Its' slow but I've used it on my 100 Ah LiFePO batteries for the RV. It takes all day/night but I do it once a year (winter) and write the tested Wh on them so I can see how they are holding up over the years.