Recommendations for BMS LiFePO4

A

Alex1

10 mW
Joined
Feb 18, 2013
Messages
28
I am looking to buy BMS for LiFePO4 48V 15Ah battery pack for my bike. Max drain current is 35A. I intend to charge batteries with 3.5A at 3.6V per cell. chargers max voltage is 57,6V.

Some recommendations for good BMS?

Did somebody tried this:
http://em3ev.com/store/index.php?route=product/product&path=42&product_id=66

Aslo how bms behaves while charging with motor brake (regentarion) I dont know how high current are there.
 
yes, that is a good one. i call them the 'headway' BMS because they are used by headway but not made by them but they are one of the best BMSs you can buy and paul has them cheaper than anyone.

you will need to increase your charger output voltage up to 59V or so to make the pack balance. 58.4 is fully charged and you need a little extra to force all the cells to fill up and balance together.
 
On the other hand, if you practice a conservative depth of discharge, you might find the pack never gets out of balance, and it doesn't matter at all if the pack charges to 57v. 57v sounds like a lead charger, and you really should have a lithium type. In a pinch you can charge with a lead charger, but its best not to use one exclusively.

But at some point, you will need it to balance, and need the higher voltage. You sort of need it right away to get that initial balancing, or you could parallel all the fully charged cells before you connect them up to get balanced. If you chose a ping type signalab bms you would need even higher voltage charger for the bms to work it's magic.

Look inside your charger, it might have a pot allowing you to adjust the voltage up a few v.

If you are building the pack yourself, add some 8s jst balance wire plugs spliced into the wires leading to the bms. It will allow you to monitor voltage at cell level quickly and easily with a cellog 8.

Ask Paul about the bms's ability to tolerate regen.
 
Thank you for your answers.
Charger has a potentiometer for adjusting output voltage up to a 60V+, so this is not a problem, and this charger is for LiFePo4 and SLA.
I have ordered 40155 for my pack.

Isnt it better not to charge LiFePO4 to 100%, and disharge them to 20%?
This bms balance it at 3.65V, and that is 100%, is it posible to lower it on 3.6V?

@ dogman

This?
http://www.hobbyking.com/hobbyking/store/__10952__Cell_Log_8M_Cell_Voltage_Monitor_2_8S_Lipo.html
Isnt there a 16M modul for 16S battery pack, to see al cells voltage at the same time?

when I recive betteries, i will connect them in paralell and balance them with dc-dc step down modul @ 3.6V
 
Yeah, that's it. Very handy things to have. Check all 16 cells in about 30 seconds.

Just wire the pack with 8s jst plugs, that is 9 wire jst plugs. then you can either use two, or just move one as needed to check cells. Don't leave em plugged in all the time though, to avoid the device causing an imbalanced pack.
 
i found the cellogs seem to use about 10-15mA. i finally wired up my cellogs on this headway pack i had over discharged in series with the ping that blew up.

i left the 8S cellog on the first 8 cells and nothing on the last 4 of the 12S pack.

after about 50 hours being left on, it took about .6Ah of charge to bring the low cells in the group of 8 up to the same charge level as the top 4 so i used that to calculate the rate the cellogs drain down the cells.

the 9 pin JST-XH harness is not too common and of course you cannot buy it at hobby king. this is what i found on ebay:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=111026744918

if you leave the window open on the cellogs at hobby king they will offer a discount after a few minutes for $13.69 or something like that.

i thought they drained current only from cell #2 but it seems like the entire group of 8 was drained by the same amount when i was recharging it.

the cellogs do make it so easy to measure the cell voltages.

i just soldered the leads onto the pins of the sense wire plug on the underside of the pcb. remember if you use two in series then the ground for the top set of 8S cellogs is actually the top of the #8 cell so you solder the two leads together there.
 
Is it possibly to use cell log instead off BMS to monitor voltage of the cells, and manualy stop discharging when one cell gets too low.
If I see correctly this 8cell log is for 4.0~43.0VDC, my pack is 48V nominal!
Is cell-log 8M powered from 4-43V? if yes, I can buy dc-dc step down module to get lower voltage for supplying two Cell-Log 8M for monitoring all 16 cells at the time of discharge/charge.
Or is it powered from one cell while measuring voltage?

Also i have found some cheep bms from china.
http://www.mistertao.com/taobao-products/taobao-item-16023606815.html
"discharge can be connected with the mouth, can also be sub-port access"
there are two wiring diagrams.
2.16 serial port connection
216serialportconnection.jpg

1.16 string sub port connect
116stringsubportconnect.jpg


This two rectangles on the right, one is charger, other is load?
I need connection with which I can charge batteries with charger and motor (regen)
Maybe I will try with cell log + this bms, insted of expensive bms.
Can anyone help?
 
48V means 16S of lifepo4. the battery labeling is a holdover from when people used SLA batteries and everything was 12V increments. cells in series.

now for lithium ion storage cells 24V is 8S or 8 lifepo4 cells in series and 48V is 16 cells in series.

so we use 3V for the nominal voltage of each cell. but the lifepo4 is fully charged when it gets to 3.65V and is almost discharged if it is at 3V.

the BMS will balance the pack for you and protect it from being over discharged by accident. it happens all the time.
 
Alex1 said:
Is it possibly to use cell log instead off BMS to monitor voltage of the cells, and manualy stop discharging when one cell gets too low.
If I see correctly this 8cell log is for 4.0~43.0VDC, my pack is 48V nominal!
Is cell-log 8M powered from 4-43V? if yes, I can buy dc-dc step down module to get lower voltage for supplying two Cell-Log 8M for monitoring all 16 cells at the time of discharge/charge.
Or is it powered from one cell while measuring voltage?

Also i have found some cheep bms from china.
http://www.mistertao.com/taobao-products/taobao-item-16023606815.html
"discharge can be connected with the mouth, can also be sub-port access"
there are two wiring diagrams.
2.16 serial port connection
216serialportconnection.jpg

1.16 string sub port connect
116stringsubportconnect.jpg


This two rectangles on the right, one is charger, other is load?
I need connection with which I can charge batteries with charger and motor (regen)
Maybe I will try with cell log + this bms, insted of expensive bms.
Can anyone help?
Click to expand...


The rectangle on the left, with 3 Chinese characters, is the charger. The rectangle on the right, with 2 Chinese characters, is the load.
The BA with Chinese characters, is the battery plug BA.
 
Ok, thanks for help.
I dont understand what is the point of sub port connect?
In my opinion with serial port connection charging and discharging is possible through load ( regeneration with motor) but is there limit current for charging battery?
Am I right, if someone know?
I will try to ask seller.
 
since this has a separate charging port i would expect it also has a set of mosfets for charging and another set for discharging. if you can find a picture of the back we can tell you.

you can do regen back through the discharging port which is the one in the middle.

usually charging should be .5C for lifepo4 pouches.
 
circuit said:
Anyone is able to read what are those 4 chips?
Click to expand...

My guess is opto-isolators. I'm using LTV-845s (4x optos) in my BMS design and they look very similar. They're to isolate the hvc/lvc signal from the individual cells below and the fets above.

Also, I'd be suspicious about whether or not that BMS is actually doing any useful balancing. Unless there are some power resistors on the back or it's using a charge shuttling technique (unlikely at that price point) then the "balancing" may just be a few mW being dissipated in the HVC circuitry. Sure it may balance the pack eventually, but it might take days on the charger to do so.

[Edit] I opened the link to that BMS and there are indeed what look like some larger resistors on the back (with the blue LEDs). Still, they look pretty small.

In general I've found that you get what you pay for with a cheap BMS, whether chinese or not. After I had one of them over-discharge and destroy one of my packs I decided to design my own. If you're interested I'm hoping to have all the kinks ironed out in the next month or two. Similar to the goodrum/fechter design I'll be making PCBs and a BOM available to anyone interested, and perhaps even some already assembled if the interest is there.
 
thanks for pulling that up.

the led is all i see on the back side. it has a current limiting resistor but i see no other shunt resistors. i think the mosfet at the bottom of the row on the front side is used to switch that led on and off when the channel is full.

there are a lotta caps all lined up along the row of through holes coming from that big 24 pin IC above on the front side. that chip you though had a collection of optos is instead some kind of switching circuit i think. i think it may operate like some of the balancing chargers that send pulses of current down each of the sense wires to balance.

the chips have a ground in the upper left corner and get power for the chip from the top of each 4 cells so it may be an integrated circuit specifically designed to force current into the low cells and not into the high cells as it is switching the output current on those pins the caps are tied to. which is why they have the caps imo.

you should still be able to charge through regen but it will not be able to balance and i expect that the HVC is not the same as the bleeding shunt type balancing BMSs. so a long regen when fully charged would be an unknown as to whether the output mosfets would turn off at HVC.
 
I have my suspicions that they are using the LED/current limiting resistor as the current bleeding resistor in order to sell this as a "balancing" BMS. Fine if the charge current tapers off at the end and you don't mind waiting forever to balance, but most people aren't going to want to wait that long. Even at a forward current of 25mA through the LEDs, a relatively small 100mAh imbalance would take ~4 hours to equalize.

From reading the specs (or at least trying to decode the translated chinese) it seems that they're saying the balancing shunt current is 60mA and either turns on at 3.6V or 3.75V and shuts off at 3.55V. At 3.75V that's just under 1/4W which should be dissipated easily enough by an LED and an additional shunt resistor, or perhaps by the base resistor on a BJT supplying the LED if they set the base current high enough. That said, I only see 2 ICs, one of which I bet is a TL431 and the other a voltage supervisor IC of some sort (TC54, NCP30x, etc).

Short version of my opinion... I think it would take forever to balance your pack with this thing if you don't do some cell damage before it's done. Even if it really can do 60mA bleed current that's an awfully slow way to balance anything over a few hundred mAh.

I'd steer clear of this one unless your BMS budget is low and your replacement cell budget is high.
 
that led is not 25mA. more like 2mA. maybe the resistors are there but i did not see them when i blew up the image. i still think it is switching the charge inside that IC and the comparator at the bottom drives the mosfet that turns on the led. you can see the comparator legs going right onto the mosfet legs. one on the source and the other on the gate.
 
I posted my reply before reading yours dnmun... I was being optimistic on the LED current.

I think you might be right about the 4 big ICs doing charge shuttling. That said, I've experimented with doing this in the past (using a LM2663) and unless the imbalance is large it takes forever, although it does work. Perhaps what they're doing is leaving the charge shuttling circuits on continuously in conjunction with a weak cell bleed through the LEDs/limiting resistors. Large imbalances would even out over the long term and small end of charge ones could be taken care of by the LED/resistor. Not the best way in my opinion, but I guess it would work. I don't really believe the 20uA quiescent current claim if this is what they're doing.
 
I do wonder how they're implementing the LVC then, perhaps hidden where we can't see on the bottom, but I believe the 5 pin IC is a voltage supervisor IC and the 3 pin one immediately below it is a TL431. The TL431 can sink about 100mA, so it would be plenty to turn on an LED.
 
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