Balancing 4S x 140ah lifepo4

[yopappamon]

I've made a 12v battery pack for an inverter out of 20ah thundersky batteries. 4 by 7 cells. I'm going to use a cellog8's alarm output for lvc to turn off the inverter when the cells go low. Possibly a second cellog8 for hvc.

I want to charge the cells with a 120w solar panel and am evaluating my options to keep the cells in balance.

On 140ah don't you need a larger bleed off current then say the Signalab provide?

I could get the balancing modules from elite power solutions where I got the cells. I don't know how expensive they are.

I'm looking for suggestions, recommendations.

 

[amberwolf]

The amount of bleed off ability you need depends on the charging current you provide. As long as the bleed capability is greater than or equal to the charge current, it's ok. Otherwise the charge current will be raising the voltage of already-charged cells despite the bleed, and eventually (or quickly, depending on how much current) damage cells.

 

[yopappamon]

The charge current would be capable of around 6-7amps. That should taper off as the cells get full, right?

The regular BMS balancers only drain around 100ma but can charge many times that, 10-30amps. How do they keep cells from going too high then?

I'm sure if the cells were way out of balance it would potentially over volt a cell or two.

I went ahead and ordered the 60ah balancers modules from Elite Power Solutions. They were only $9 each and will balance up to 0.33amps. If I need more I might could mod them to an external resistor bank or something.

 

[amberwolf]

Assuming a nearly-balanced pack, then yes as the charge tapers off then the current should drop to where balancing can handle it.

But as you say, with an unbalanced pack that has cells quite full when others are not nearly so, keeping the pack voltage low relative to charge voltage, and thus charge current higher, it'd overwhelm the balancers and those cells'd overvolt, which is where damage can be caused (and eventually would be, if not immediately, depending on how bad it is and for how long).

 

[yopappamon]

So you are saying I either need a balancing current equal or larger than the charge current or a cell HVC.

 

[AndyH]

Sounds like I'm picking up some of your brain waves, Yo!

I've got a couple of different 100Ah LiFePO4 cells and some other bits and plan a 'portable' power cart. Initially I'll connect a power strip and use it to charge all the portable items and run the NiMh charger for all the AA cells in the house. I'd like to grow it so it'll run my office 24/7. We'll see how that goes.

If you find that you need more shunt, I can help. I'm playing with one of Gary's creations. The 4S 12V board was intended for a 12V starter battery replacement, but I haven't yet gotten the automatic disconnect to work. The shunts, though, are capable of more than 1A (should be closer to 2) and they work well. There's no charge control or HVC so the charge current needs to be lower than 2A to keep cells from getting too far out - but this can be offset by charging lower than the 3.7V shunt activation. It can't handle a very out of balance pack, but does a great job on a relatively healthy pack. It also works well as a parallel shunt to augment a smaller BMS.



The second board is a commercial product. The basic board is a 4S unit with 200mA shunts. It was designed for a 40Ah max pack. The additional board is an active cut that works for both charging and discharging. It'll handle a 30A continuous (60A peak with a heat sink) load and 15A max charging. It appears that it will throttle the charge current - I did a test last night with a 2A charger and the active board throttled the end of charge current down - and turned it off when necessary - to allow the pack to balance without overrunning the shunts. In addition, it has a USB output that allows cell voltage monitoring, and retrieval of over- or under-voltage error events and pack cycle count.



Last toy is an Arduino. The mail man brought a pair of these yesterday: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=230584210368 They're isolated bidirectional 100A current sensors designed for direct-connection with an Arduino. The board should allow plenty of data collection. A couple of relay controls might also allow disconnecting the load or solar panel on HVC/LVC events.

Better yet - you mentioned CellLog. What do you recommend for an inverter? Are you using a charge controller?

I followed Jeremy Harris' lead and ordered a FatMax MPPT board from Oz. I look forward to playing with this as well.
http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=18125&p=304143#p304143
http://solarmppt.com/
http://www.solarfreaks.com/fatmax-high-power-maximum-power-point-tracking-unit-t142.html

Fun!

 

[yopappamon]

Sounds like I'm picking up some of your brain waves, Yo!

I've got a couple of different 100Ah LiFePO4 cells and some other bits and plan a 'portable' power cart. Initially I'll connect a power strip and use it to charge all the portable items and run the NiMh charger for all the AA cells in the house. I'd like to grow it so it'll run my office 24/7. We'll see how that goes.

Better yet - you mentioned CellLog. What do you recommend for an inverter? Are you using a charge controller?

That's what I'm building, a portable power cart. I currently have a cellog alarm output ran through an opto isolator (probably don't need it, but didn't realize it until too late), firing a FET to switch off the battery feed to the inverter on/off switch. This will be my LVC. I got the inverter from Harbor Freight on the advise of others here. Very inexpensive. I just ordered 4 balancer modules to keep the cells in balance. I'm thinking I'll add another cellog as a HVC, run the alarm output to another FET to switch off the solar charge feed.

I have a cheapy solar charge controller for lead acid battery. I'll see what the output voltage is for that and see if it will work for me.

That Hall current sensor is very cool!

I need to figure a way to reset the celllogs once they go into alarm. I think they latch.