Charging with higher voltage charger

Batteries, Chargers, and Battery Management Systems.
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HrKlev   10 mW

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Charging with higher voltage charger

Post by HrKlev » Mar 25 2020 3:55am

Hi

I have just finished a 16S6P pack that I would like to run through some testes before I wrap it completely up. The pack includes a bluetooth BMS where I can set pretty much most of the impotant parameters, and monitor each cell group. My 16S charger is of course stuck somewhere on its way from china.

My question is, what happens if I plug in a 20S 5A charger I have laying around here? Will it happily deliver 5A at 52V, or will magic smoke appear from the charger? I will set the cut off voltage in the BMS to around 4.1V per cell and stick around while monitoring through the BMS app. It wil just be for testing purpose to finish my pack build, and I wont be using the pack on a regular basis before the 16S charger appears. Not risking the fire if the BMS fails.

Asking for a friend, of course

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dogman dan   100 GW

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Re: Charging with higher voltage charger

Post by dogman dan » Mar 25 2020 6:13am

Uhh, you can, but its risky. One time to test stuff would be the max I'd advise doing it, and stop early manually. REALLY early, like 3.8v per cell.

The charger will try to put 72v into it, fast. So make each connection brief, just enough to put enough energy in to test the motor systems.

You can use some other charger, such as a 8s RC charger if you have one, connected at low amps through the correct two balance plug contacts. Charge each 8s, or whatever, section one by one.

Jon NCal   1 kW

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Re: Charging with higher voltage charger

Post by Jon NCal » Mar 25 2020 8:59am

Could you adjust your 20S charger to correct voltage for the 16S battery?

For example, I was able to adjust a 10S 42v kingpan type charger for use on 12S and 14S batteries.

john61ct   1.21 GW

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Re: Charging with higher voltage charger

Post by john61ct » Mar 25 2020 9:24am

The actual voltage of the combined circuit gets "negotiated" between the battery (SoC, resistance) and the source (current rate).

At a high C-rate the source has more influence.

So long as you or a HVC actually terminate the charge cycle before a harmful overvoltage shows at the battery terminals, no problem.

Then let the battery sit isolated, test its resting voltage for SoC, repeat a bit more, etc

Just know especially at a high current C-rate and/or worn cells, this can be risky, damage the pack or even start a fire if you aren't sitting there monitoring closely, watch like a hawk.

HrKlev   10 mW

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Re: Charging with higher voltage charger

Post by HrKlev » Mar 25 2020 9:50am

I doubt I can adjust the charger voltage that much. I have another 20S 10A charger of the same brand, xyz charger (aliexpress), and it could only be adjusted by a couple volts... I will open up an check.

The charger Im thinking about using is a 5A one, and the battery consists of 6P groups of 2500mah cells, so C-rating is low, around 0.3.

I think I will give it a shot this evening. Im always too impatience...

john61ct   1.21 GW

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Re: Charging with higher voltage charger

Post by john61ct » Mar 25 2020 10:36am

Use an accurate voltmeter

Ideally stop based on the per-group voltage, first / weakest one shuts down the cycle at say 4.1V not whole-pack voltage.

If the latter maybe use 4.05Vpc

Better to stop too early then add a bit more

HrKlev   10 mW

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Re: Charging with higher voltage charger

Post by HrKlev » Mar 25 2020 1:01pm

Well, nothing went boom at once, at least 😃

I'll be watching it carefully, but it seems fine so far.
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Hillhater   100 GW

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Re: Charging with higher voltage charger

Post by Hillhater » Mar 25 2020 4:08pm

You are fortunate in one respect....
Many chargers wont even commence a charge if the voltage os below a preset level for the range of the pack its intended for...
Which , if yours had that protection for 20s that would have been somewhere in the low 50v range !
This forum owes its existence to Justin of ebikes.ca

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amberwolf   100 GW

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Re: Charging with higher voltage charger

Post by amberwolf » Mar 25 2020 9:49pm

the safety problem with doing this is if your bms does not stop the charge for whatever reason, you'll overcharge the entire pack.

20s charger should be around 84v output for a full charge.

84v / 16s = 5.25v per cell.

that's a hell of an overcharge if the bms fails to stop it, which will destroy your brand new pack and require you to toss it out and start over.


also, one great way to damage a bms so that it will allow this to happen is to put too high a voltage on it's input (or output), which will damage the fets that block the charging (or discharging). the most common failure mode for fets is to fail shorted, meaning the bms would no longer be able to turn off charging (or discharging).

if the fets are not rated for at least 84v, they could be damaged in a way that prevents the bms from disconnecting the charger when cells reach full.

when that happens, the charger will keep going and destroy the cells.


also, if it does destroy the cells at 5.25v per cell, that voltage is also present on all the bms sense lines. if they aren't built to take that much voltage (since they only need to take a little more than 4.2v) it could damage those, too, so you'd need a new bms and new cells.


in either case...you'll be starting completely from scratch on building your battery. :(

john61ct   1.21 GW

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Re: Charging with higher voltage charger

Post by john61ct » Mar 25 2020 11:05pm

My instruction was sitting watching the voltage like a hawk, "live" so as to stop before the Full voltage is reached.

Certainly not relying on an untested device to HVC

HrKlev   10 mW

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Re: Charging with higher voltage charger

Post by HrKlev » Mar 26 2020 2:29am

Im aware of the dangers, and I will not be using this on a regular basis. Pack was sitting on a concrete floor with me within 5m regularly checking the BMS. When the cells reached 4.1v, the BMS stopped the charging as I programmed it for. The cells settled just above 4.0v, and in perfect balance, so I have done all the tests I need, and will be wrapping the pack up next. I wont be charging it anymore until I get the 16s charger.

Thanks for the heads up on the mosfet voltages, Amberwolf. I will check if they are still working when I get the 16S charger by turning them on an off some cycles.

Its a good thing with all the warnings on this thread, as it is a big safety concern doing it like this. For any kind of regular charging there should be at least two barriers, BMS and charger. For this experiment I was the second barrier, sitting there right next to it.

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dogman dan   100 GW

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Re: Charging with higher voltage charger

Post by dogman dan » Mar 26 2020 5:48am

Still very likely that its charging the thing faster than is good for longevity. That's why I suggested just enough charge to allow a test of the device the battery is for.

I would not trust that bms.. But that's me, I'll never, ever trust a bms again. Not inside my house.

john61ct   1.21 GW

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Re: Charging with higher voltage charger

Post by john61ct » Mar 26 2020 8:55am

Maximum 0.3C rate, can't get "too fast".

Measured voltage never got above 4.1Vpc on the circuit, could not have ever been higher inside the battery.

leffex   100 W

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Re: Charging with higher voltage charger

Post by leffex » Mar 28 2020 3:19pm

HrKlev wrote:
Mar 26 2020 2:29am
Im aware of the dangers, and I will not be using this on a regular basis. Pack was sitting on a concrete floor with me within 5m regularly checking the BMS. When the cells reached 4.1v, the BMS stopped the charging as I programmed it for. The cells settled just above 4.0v, and in perfect balance, so I have done all the tests I need, and will be wrapping the pack up next. I wont be charging it anymore until I get the 16s charger.

Thanks for the heads up on the mosfet voltages, Amberwolf. I will check if they are still working when I get the 16S charger by turning them on an off some cycles.

Its a good thing with all the warnings on this thread, as it is a big safety concern doing it like this. For any kind of regular charging there should be at least two barriers, BMS and charger. For this experiment I was the second barrier, sitting there right next to it.
To answer your question. Yes you can and it works but then its risky and dangerous if not "looked after" properly in a sense the way you did by being there and checking it every 5 minutes
.
Hehe yes. You did great. and yes it is dangerous! Even I checking and being present overcharged my batteries and I had to do a run on my bike to empty the batteries FAST. The danger was 4.35v charged per cell and I did it twice 2019 but then I fixed the problem because I didn't want it to ever happen again.

Before that I warned my brother not to charge 4 packs but only 5 packs in series. However he did charge them in that way anyway so hi cells were sitting at 4.4v per cell on a rc lipo battery and I didn't know for how long!!!

Second time . He was home around taking it easy and having a blast for a day with pizza. By then I had told him several times to charge properly every time I visited. However he decided to charge faster and just put 4 packs in series for the 5 pack series charger(20s, he was charging 16s). Well, the first floor balcony started to BURN and there was fire and he somehow killed the fire fast and now, my brother, real world electrician is very afraid of ebikes.

To tell your friend its ok. IT IS NOT OK or your brother or your smart electrician friend. They do not know ebike batteries and the danger. The danger is by far most probable when charging. That's when the most damage can happen.

Keep safe!
Salestech in an ebike store

john61ct   1.21 GW

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Re: Charging with higher voltage charger

Post by john61ct » Mar 28 2020 7:06pm

Voltage based relays as HVC really aren't rocket science nor expensive.

Put two in series for redundancy, can then use on any rectifier / PSU, DC converter or untrusted charger

As long as the C-rate is not too high.

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