Discharged to deep (LiPo pack w/ bms)

[underdog]

Hello.

I have a little challenge in my hands. I discharged my 2P20S LiPo pack to deep for my charger to start charging it. I removed parts of the enclousing tape to disconnect my BMS. I found that all the parallels are between 3,50 and 3,66v To a total of about 72,1 volts. I figured that this is a product of my BMS cutting the load at a lower voltage than my charger will accept to start charging my pack with 84volts.

I had the pack inside over night to get to about 20C temp, hoping this vould increese the volt to a acceptable level for the charger. Sadly it did not. Next sted would be to heat it to 40C temp in the oven. But some how I do not like the sound of putting a big LiPo pack in a owen, not knowing if that 40C on the knob will actually be way above at places nearer to the elements... :?

Sadly I have soldered all the main power-cables together between the packs, so I can not get the main connectors from the balance charger to the + and - directly. All I can tap into without dismantling large amounts is to charge via the 24AWG balance cables. But I find that to be a not so good solution, since that will take days, brining into account that I dont tink anything over 0,6A sould be a good idea, and my balance charger only does "full-charge" mode 4,2v. If I could only bring them up to 3,7, that would work at that low amp i guess (taking time into consideration).

Does anybody have any good ideas of how to soulve this in a timly fashion without doing dangerous manuvers to the pack?

 

[ARod1993]

I would be incredibly uncomfortable with the idea of throwing a LiPo pack in the oven ; even though the oven would be decently able to contain a LiPo fire it would also be fairly likely to instigate a LiPo fire. Just out of curiosity, do you know what your charger's minimum turn-on voltage is? If you can start charging via the balance leads for several hours, then pull the pack off after the cell groups have hit 3.7-3.75V or so average and reattach the bulk charger then you'd be in pretty good shape without having to do drastic things to the pack. Barring that, if you can find someone who has (or find for yourself) a bench supply capable of outputting 0-100V at a respectable output amperage then you'll be able to work with that.

Basically, buying something like this (or this if you need to save money and are willing to take a mild to moderate amount of risk on an eBay listing from a company I know nothing about) would do you quite well. With one of those supplies, you can just set the output voltage to 83.5-84V and the maximum current to some reasonable single-digit number of amps (assuming you're using RC LiPo packs or some other battery chemistry that can handle >0.5C-1C charging) and plug it in. The supply will provide current at the limit until the battery voltage hits whatever you set the power supply to, and then it'll just maintain the charge voltage as current trails off; it's basically the same as a manually set CC/CV bulk charger, only a lot more versatile (and somewhat easier to blow up your battery with if you're careless).

 

[izeman]

i don't think i really understand your problem. if alle cells are above 3.5v then this is the perfect lvc voltage. no problem here. if your charger doesn't start at that voltage there's something wrong with it. normally they stop charging anything below around 2.7v. so i would look for a different charger IF what you say really all is true and tripple checked
charging through the balance leads is a common method. search for bc168 on the forum to learn more. if you really need to bring voltage to a higher level just build a charging lead with banana plugs on one end for the charger and a 2pin header on the other side to plug into the balance plug. charging with 3a is a safe current if your balance lead are 22awg or thicker. charging should not take more than a few minutes for every cell to bring them to a good level.
i don't think that putting your lipo in an oven is a usefull idea but it definately won't harm them to go to 40c.
good luck!

 

[dnmun]

i see no evidence it was over discharged. if there is a BMS on the pack then just put the bulk charger on it and it should charge up. if not then there is something wrong with the charger or one of the sense wires is disconnected and shut off the mosfets. check the gate voltage on the output mosfets and post up a picture of your BMS.

 

[izeman]

yes. a picture/type of the battery, charger and bms would maybe help.

 

[999zip999]

Check charger voltage.

 

[wesnewell]

For all you people that don't know what a gate is in electronics, this might help.
http://en.wikipedia.org/wiki/Logic_gate

 

[dnmun]

the mosfet gate has nothing to do with a boolean function.

it controls the conduction in the channel of the mosfet. measure the voltage between the gate and the source legs.

on the regular TO-220 package the legs go: gate, drain, source, and you wanna know what the voltage between the gate and source is.

 

[wesnewell]

It would serve you better to read the link than just may some snide remark. For those that want to know the construction of a mosfet so can check the gate, this will help.
http://en.wikipedia.org/wiki/MOSFET

 

[underdog]

Oki. Little update.

I am seeing some wierd behaviour from my BMS system I think. The pack is constructed of The Good-priced 4s 20C Hardcase LiPo`s from HK. Two and two put upon each other and 12awg leads soldered together into 8awg. It has a D131 80A BMS and closed off with supersticky tape. 74V 10Ah.

Edit: Charger is a cheepo BMSBattery EMC-400, 84V 5A, alu enclosed.

I went on a long distance trip on saturday without checking my tyre preassure. Suddenly after about 30 km under a heavy acceleration pulling about 50-60A I ran out of juice and my CA shut down by the voltage dropping like a rock down to 14v. I figured that the BMS must have cut the load and what I was seeing on the CA before going black was the capacitors in the controller charging out its last current to the CA. I belive this BMS from Bestech should recover automaticly, but I had to disconnect the whole pack and leave it for half a minute before connecting it back up. This turned everything on again, and I realized that my pack voltage was about 72V sitting. So I tried to pedal on, and realized that I had just about 0,5 bar in my tyres. Heavy bike + low preassure + 1 speed = impossible to pedal. So I pushed to the nearest gas station and filled my tyres. Eventually managed to get where I was going by limiting current to 5A on the ca (200-300W).

Next morning I put it on the 5A charger for 90 minutes, and checked the voltage to be only 75v, and unconfidently started my 40 km trip home. Got half ways and found a Tesla-charging station and gave it another 45 min on the charger. Got home from there, and parked my bike. It would not charge. So I took the pack inside over night and next morning it still would not charge.

Then I disconnected the BMS from the cells and checked the votages and made this post. Put the Pack on my bathroom floor and turned on the floor heating for some hours. Then I connected my BMS back on and still would not charge. Then suddenly without doing anything else than to try once more later on the day, the charger started without hesitation. Charged it up and all good.

But I have to say I dont like not knowing what caused the weird behaviour. Reading your help, I understand that the charger did not start because of the BMS would not let it. Then suddenly it would... Will a BMS disconnect the charge-circuit with a Voltage-delta of only 0,16 V between cells perhaps? I am very cautious with my LiPo`s so getting to the bottom of this behaviour would give me greate peace of mind..

 

[dnmun]

you will have to measure the cell voltages if you don't have cellogs on your pack.

when you charge the lipo D131 up to the point where one of the cells goes to the HVC and turns off the charge, it has to drop in voltage down to about 4.05V before the charging mosfets will turn back on again. this is to protect the pack from overcharging and is different from the type of behavior that the lifepo4 BMS exhibit.

if you can measure the individual cells then you will have a better idea of what is happening.

 

[999zip999]

Plus knowing the cell voltage under load not just at rest.

 

[dnmun]

i finally looked at the picture. duh..

where are the sense wires?

 

[izeman]

where are the sense wires?

that's what came to my mind when i saw the picture as well. it looks like the balance wires are NOT connected to the bms. i wonder how/why it charged even once?! that's maybe a strange behaviour of the bms as i mentioned before in other threads. lose balance wires will NOT trigger a dis/charge, therefor you can never be 100% sure that everything is working ok. but i can't imagine that the bms would allow a charge w/o ANY balance wire connected. i should try that out (so it never came to my mind that it could work though *lol*)
edit: maybe the balance wire harness was removed for measuring cell voltages and not put on again while taking the picture???

 

[dnmun]

no, it will not work without any sense wires attached imo. it needs a voltage on the 3rd sense wire as well as the ground to create the circuit current to drive the mosfets on the output.

 

[underdog]

where are the sense wires?

that's what came to my mind when i saw the picture as well. it looks like the balance wires are NOT connected to the bms. i wonder how/why it charged even once?! that's maybe a strange behaviour of the bms as i mentioned before in other threads. lose balance wires will NOT trigger a dis/charge, therefor you can never be 100% sure that everything is working ok. but i can't imagine that the bms would allow a charge w/o ANY balance wire connected. i should try that out (so it never came to my mind that it could work though *lol*)
edit: maybe the balance wire harness was removed for measuring cell voltages and not put on again while taking the picture???

Ok, I understand that I should have mentioned that this is a picture where the sense-wires are disconnected. I thought people would understand that I do not run the BMS without these. Just did not have a good picture of it complete connencted.... :wink:

 

[dnmun]

the picture was all we have. it seems obvious you don't understand how the BMS works so we were trying to help you by asking for the cell voltages while it is charging.

heating it has nothing to do with anything and the idea that somehow the BMS can measure the final voltage of the pack to allow charging is also false. i am sorry people can mislead you into silly ideas but i cannot keep people from posting whatever pops into their mind.

i explained the only reason that the BMS will prevent charging but we still have no cell voltage measurements or immediate history preceding this situation you have encountered.

 

[underdog]

the picture was all we have. it seems obvious you don't understand how the BMS works so we were trying to help you by asking for the cell voltages while it is charging.

heating it has nothing to do with anything and the idea that somehow the BMS can measure the final voltage of the pack to allow charging is also false. i am sorry people can mislead you into silly ideas but i cannot keep people from posting whatever pops into their mind.

i explained the only reason that the BMS will prevent charging but we still have no cell voltage measurements or immediate history preceding this situation you have encountered.

Misinterpret me right I am happy for all help I get. I am sorry for misleading with the picture.

I do now understand that the reason my charger did not start is still a mystery... I am not paranoid, but I feel that if one should posess lipo`s, one should understand and account for every situasion in the goal of beeing on the safe side. I am not a electronics genius (yet), but I have a big eager to learn.

 

[dnmun]

do you have a voltmeter? do you know what the mosfets are on the power board on top of the BMS?

there are two sets. 5 for the charging connected at the C- spot and then there are 15 around the rest of the power board that are the output mosfets.

there is a heat sink on top of them and you only see the legs. the legs are gate, drain, source from left to right, on each mosfet.

all 5 of the charging mosfets are wired in parallel and the other 15 output mosfets are wired in parallel. you only need to measure the gate voltage on one of them to know what all of them are doing.

use the 20V DC scale. put the black probe on the source leg and the red probe on the gate, and measure the gate voltage to see if it is turned on. for both sets.

your sense wires need to be plugged in for this. lemme know what you find out and we can go from there.

there is not much more risk with the lipo than with other batteries if you have a BMS to protect it. the biggest risk with all these batteries is shorting them out and that is where the risk is. they do not over charge when under a BMS or a balancing charger. bulk charging with no protection can lead to overcharging but since you have the D131 that is not a risk you have.