Balanced charger, or BMS.

[audia2ev]

Reading through the pages on BMS here and elsewhere has got me thinking and I guess what I am going to ask will have a range of answers. Or maybe not...

My battery pack will be built out of 95 HiPower 50Ah LiFePo4 cells, in series, giving a nominal 300V pack. I purchased them with the HiPower charger, which includes balancing, when charging. Seemed a good idea since I figured I was mitigating problems that might ensue re: warranty if I used another charger, and their charging system did balancing.

Of course the balancing system doesn't operate on use/discharge. Moreover, since the system I am building should be capable of regen (DMOC445 controller), any regen won't be controlled by any BMS.

I guess the two most obvious risks are 1) low voltage on a cell, and 2) overvoltage due to regen. I suppose there are other parameters that a BMS would monitor but maybe (?) these are less fundamentally important.

The question is: is it seriously unwise to try and run a pack like this with effectively only half the battery management active (i.e. during charging)?

Given the pack capacity - 15kWh - and my expected daily use - probably 2 to 5 kWh - I expect to keep the battery state to between 30% and 80% charge. (I see that such claims have led to pretty loud exchanges elsewhere but doesn't seem an unreasonable hypothesis to me today). Obviously there will be a pack monitor to give the global state (I have purchased a Link 10). Regen can also be dis-activated by a dashboard switch.

Is that adequate to give a reasonable level of protection concerning low or over voltage? yes, one can screw it up and let everything get too low or regen and overload... but...

If not - what should I be looking to add in in terms of BMS?

Thanks in advance

Simon

[retepsnikrep]

IMHO you do need a BMS for peace of mind there are lots to choose from.

http://liionbms.com/php/bms_options.php

If you don't have one you could damage a cell by over or under discharging it during use.

Having said that if the supplied charger equalises all the cell by the end of the charge cycle, and you then draw 30-50% of capacity and don't use regen for the first mile or so, you may get away without having one

But if you go outside these parameters and need to push the vehicle to the end of it's range then you may damage a cell if you don't stop driving in time. You will never know the maximum range on any given day if you have no way of detecting when the first cell reaches empty.

[GregsGarage]

Hi Simon,

You are right in thinking that your charger isn't the whole solution. I expect that your controller can be set with min and max pack voltages, so that should help some. You could set it so that min pack voltage is 199.5 volts (2.1vpc) and max 356 volts (around 3.75vpc) and as long as your pack is reasonable balanced that should give some protection. But you really should have a cutoff if any cell goes above or below its allowed limits (2.0 and 3.85 for the hipower cells).

Peters slaves provide 2 outputs. In digital mode one output provides cell data and the other provides slave bus communication. In analogue mode one output provides a low voltage warning and the other high voltage warning. The digital mode requires a master control board and the analogue mode is intended as a stand alone system. At the moment both Peter's and my systems are operating in analogue mode and only give us a warning but won't actually stop us from driving.

Recently Peter got the analogue software to operate on the digital slaves. This means he can switch between the analogue and digital system without having to modify the slaves or alter the wiring, just reprogram them and add the master board. I then proposed that one output could be used as both high and low voltage warning. This output could be connected to your charger and dash warning and half speed input that some controllers provide to warn and cut back power when the cells are near their limit. We are using 2.4v and 3.75 volts for the warning and charger cutback. That leaves a second output doing nothing, so that could be connected to a charger absolute cut-off (3.85 volts) and a controller absolute cut-off (2.0 volts). This system would give you a warning that cells are getting low before completely cutting the drive, which could be dangerous in a car.

Edit: If you did want to use Peters boards, then you can simply comment out the few lines regarding the load balancing.

[audia2ev]

OK that's two really useful replies and and at least for the moment makes me feel that no big mistake yet... I'll come back to this when I get to the more appropriate stage.

Ciao

Simon

[milo0105]

put the cut-off voltage at 2.6v would be good, because almost power of lifepo4 battery will be discharged at 3.1v to 2.7v. for the pack the discharging cut-off voltage is 2.6*cells. you could get more than 95% of the battery pack's capacity.