My Lifepo4 dies quickly

[JoseBcn]

I have the same battery, and I have the same problem. When I found I changed the damaged cells, but what I fail to me is the BMS. My delivery 54V battery until connect the engine, then the voltage on P + / P-BMS drops to 5V, while in B + / B-, remain 54V. If I connect the motor directly to the battery, it works without problem
I've tried with 3 BMS equal to 3 I get the same problem. I really think they are very bad.

 

[dnmun]

that sounds like a different problem. he has power briefly and you seem to indicate you had no power. did you ever measure the gate voltage on the discharge mosfets? if three did not work then i suspect the sense wire is disconnected somewhere. they are not that bad. maybe.

 

[JoseBcn]

No, i don?t mesured it

 

[JoseBcn]

No, i don?t mesured it

Whithout motor connected:
In four charge mosffets, 3,2V-3,2V
In four discharge mosffets, 3,1V-3,1V

Whith motor connected:
In four charge mosffets, 51v-0V
In four discharge mosffets, 7,8V-0,5V

 

[Jason27]

1000watt you got ripped off dude. Did you buy the pack on eBay?

 

[1000watt]

1000watt you got ripped off dude. Did you buy the pack on eBay?

yes, I 'opened a case' on ebay with seller but they are very slow to respond to resolve the situation.

 

[JoseBcn]

For what it's worth to you, I got to send me another battery because the first failed me 3 packets of cells, and 3 months later I received.
Now I try to solve the failure of the BMS in order to use

 

[1000watt]

Ok, Im back.

As you know I have a problem with the battery cutting out under small load, works fine when on a stand but as soon as I put any load onto it, it cuts out.

Im thinking it could be a bad BMS, so I bypassed the BMS and the problem seems to be fixed. I took the bike for a long ride with no issues.

Is is bad to ride the bike with the bms bypass? I am still charging it through the BMS.

 

[999zip999]

Get it fixed. You must be the BMS and monitor each cell group for voltage and balance. You must know or else.

 

[d8veh]

Ok, Im back.

As you know I have a problem with the battery cutting out under small load, works fine when on a stand but as soon as I put any load onto it, it cuts out.

Im thinking it could be a bad BMS, so I bypassed the BMS and the problem seems to be fixed. I took the bike for a long ride with no issues.

Is is bad to ride the bike with the bms bypass? I am still charging it through the BMS.

You'll probably be OK. Most controllers have a LVC that cuts off before the BMS LVC. But it can't detect any low cells, so it's OK as long as your battery stays in balance. It would be a good idea to check the voltage of the individual cells after the first deepish discharge to see if you have any weak ones, and repeat occasionally in the future.

 

[999zip999]

I like to put a set of balance wires on so you can check with a cellog and you can bring up a low cell or drain it. But that gets old and a BMS is best with extra balance wires.

 

[dnmun]

there did not appear to be anything wrong with the BMS when balancing. it cuts out when you use it so that implies a high resistance cell is hitting the LVC. so it is not the BMS imo. too bad you cannot test it.

 

[GSW999]

Man this must be such an annoying problem , I am trying to build my own battery for this very reason ( Expensive returns to China ) , I hope you get it sorted , sounds like a lot of work.

I have bought a few things from China that have been OK , LED's etc but some stuff is just put together so badly , I have an aversion to paying £300 for a battery that is wrapped in duck tape , although would happily used a bodged one if it meant getting a result for low money.

Cheers


Gavin

 

[dnmun]

i don't think you should worry about returning a battery to china. but you will have a hard time finding a decent lifepo4 battery for 300 pounds.

in this case he finally got it to balance but still has a problem that causes the BMS to shut off quickly. we have no pictures so cannot be sure how it is wired up so it is impossible to determine if it is something obvious in the wiring but it sounds like it is hitting either the LVC on one of the cells that has a bad connection or the shunt is causing it to hit the over current limit at too low a current draw. the only way to find these things is to do a bench test to find the cell that goes low in voltage, then figure out how to fix it. but running it with no BMS when it has a problem with one cell going low just means there is no protection on discharge. at least he can charge it without overcharging the cells but the 4V+ readings imply to me that the BMS which we still have never seen is low quality. or the voltmeter is off.

 

[maytag]

After reading thru the pg.1 of this thread the first thing I thought was "bypass the BMS and see if it can make it a few blocks then remeasure the cells"

I know I'll probably get some Thumbs Downs from other ES members for mentioning this but I honestly haven't used a board type of BMS on my 20s Headway pack in nearly 3yrs. I use a pretty simple approach to manage my pack's Charging & Discharging. Before every ride I reset my DrainBrain (oldschool Cycle Analyst) and monitor how many AHs I use making sure not to go beyond 8.5AHs. Without getting into too much details of my manual BMS theory I was surprise to find this guy's blog who does EV car conversions. He seems to be using the same approach but I recharge my pack to 3.8v/cell while he's a bit more conservative limiting his to 3.5v/cell and zero cell balancing involved. Maybe after every 5-10 recharges I would measure each cell's resting voltage, nearly 3yrs later all 20 cells still rest between 3.33-3.34v . . . Mayb I've just been lucky 8)

http://myelifenow.blogspot.com/2012/10/lifepo4-charging-method-dont-ruin-your.html

 

[dnmun]

there is a difference in what happens when one of the cells in the series shorts out and dies to 0V. with a lifepo4 pack then you just get some cells overcharging, but if it is lipo then when that one cell overcharges it can end up being so overcharged that it can break out into thermal runaway.

that is why people use the balancing chargers to charge lipo. if you bulk charge lipo there is the constant risk that you will have this kind of accident. it is a small risk because the lipo cells do remain closely balanced, but the BMS also will protect the pack from short circuit on the output which can lead to fires in any type of battery.

the only lipo fires we have had are from a short circuit on the balancing leads of a BC168 and the other was from bulk charging a 35S lipo pack unattended. the other fire was not as serious and it was from plugging two lipo packs together in parallel and one was higher voltage than the other so it over charged the lower one. that was a small fire.

but i see no reason to actually avoid using a BMS to balance and protect the battery. that is why the battery manufacturers all use them. the problem is now that everyone uses the lipo from hobby king and they do not come with a BMS so people have to use the balancing charger instead of balancing with a BMS. i have only seen one 6S HK pack with dead cells that caused overcharging of another cell under bulk charging. two of them were at 0V and one was at 4.45V and swollen up like a balloon.

 

[Gregb]

I ride without a BMS but use a BVM-8S meter as protection against over discharge. It has a built in alarm that can be used to monitor LVC. I only have a 24V system so with 36V u would need 2 of these.
The display isn't as good as the cellog but I only need the red led to come on and the sound and I know it pedalling only time...... I have compared it against my cellogs and accuracy is identical..
PS u can use them to check the balance if you use a bulk charger and set the alarm to HVC