LiFePo4 starter battery BMS with only 2 leads?

helpfulguy

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Aug 15, 2013
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Hello,

I previously did this for my 400cc single cylinder motorcycle and it worked great:


  • 4s A123 26650
    No BMS

I just dropped it in, and it worked great, cranked it like nothing. I now need for my small 70cc.

Problem:
It is not uncommon for old scooters to have "leak" short circuits that drain the battery slowly. Instrument panel cables deteriorate etc. I would prefer if possible to have low voltage cut-off, to protect battery in that case. But not required.

I got this BMS: https://www.banggood.com/4S-100A-12V-LiFePo4-18650-Battery-Cell-BMS-Protection-Board-Balance-With-Cable-p-1217848.html?rmmds=search&cur_warehouse=CN

But it requires separate lead for charge and for drain. My scooter only has positive and negative, so I don't think I can use the BMS.

Suggestions?


Question 2:
Have been out of the loop for a while - have anything happened that can accomplish better, than A123 26650? I would consider factory built packs but they seem to be $150+ compared to $60 for DYI.
 
yes, you need a bms with just two leads, for it to use the scooters positive input to charge when it runs.
 
dogman dan said:
yes, you need a bms with just two leads, for it to use the scooters positive input to charge when it runs.


Thanks buddy, I wasn't even sure those existed, what term should I search for to find them? Any actual item suggestions? I need 100A I think.
 
Why you need BMS???
for 4 S. pack?
All you need is LVC
the best is or rather was celllog.
Discontinued by all sellers
It has open collector alarm output
 
They do make BMS boards with a single port. Here's an example:
https://www.ebay.com/itm/Waterproof...hash=item23aa294302:m:mspFssOoAy_6X3tjxu-U3NA

But 4s of A123 cells is probably almost the same cost. You could skip the BMS and just replace the cells if they get damaged.
You could also test with a no-BMS pack and measure the voltage periodically to see if it is dropping.
Another option is to use a high current disconnect or switch on the battery and just disconnect it when not in use.
 
It is somehow related
I built start up battery for my V6 engine Honda.
It is Nissan Leaf 4S cells.
Stay in balance beautifully ,already 4 harsh Canadian winters and going strong
 
Series connected cells / blocks cannot balance themselves.

Perhaps you mean, if there is no BMS involved, that they tend to remain in balance, do not get unbalanced in your use case.
 
Whatever you call "balanced" ...
Voltage difference is identical after every car startup and after sitting for a week.
Call It what you want. balanced?
they definitely DO NOT need any BMS.
 
Yes I've manually balanced dozens of banks, really should be done prior to paralleling for the first time.

No BMS is required, and in fact their usual approach is super slow and inefficient, can even be harmful.

But "not getting out of balance" is not "self balancing", the latter only happens to a block of cells wired in parallel.

That's all I was pointing out.
 
helpfulguy said:
I got this BMS: https://www.banggood.com/4S-100A-12V-LiFePo4-18650-Battery-Cell-BMS-Protection-Board-Balance-With-Cable-p-1217848.html?rmmds=search&cur_warehouse=CN

But it requires separate lead for charge and for drain. My scooter only has positive and negative, so I don't think I can use the BMS.

Suggestions?


Question 2:
Have been out of the loop for a while - have anything happened that can accomplish better, than A123 26650? I would consider factory built packs but they seem to be $150+ compared to $60 for DYI.
I did a car starter battery with 2 of this in parallel https://www.ebay.com/itm/4S-12-8V-1...ard-3-2V-x4-/322613685070?hash=item4b1d462b4e
They are good to start 1.6l motor sometimes, but half of the times over current shuts it down. I may add 1 more, to have 3 in parallel. On those you can use 2 or 3 wire connection, only drawback is double of heat if 2 wire used, as discharge and charge current have to cross both Fet arrays. Not much of the problem if it happens for a dozen of seconds at a time.
 
P.S. Having a BMS did already saved the battery one time as I forgot to switch off the car after doing some screwing around, then after 3 days I just got a discharged and not dead battery, happy to not have listened to those "you do not need BMS screamers" :D
 
parabellum said:
[
I did a car starter battery with 2 of this in parallel https://www.ebay.com/itm/4S-12-8V-1...ard-3-2V-x4-/322613685070?hash=item4b1d462b4e
They are good to start 1.6l motor sometimes, but half of the times over current shuts it down. I may add 1 more, to have 3 in parallel. On those you can use 2 or 3 wire connection, only drawback is double of heat if 2 wire used, as discharge and charge current have to cross both Fet arrays. Not much of the problem if it happens for a dozen of seconds at a time.

You might be able to just add some shunt resistor across the existing ones to increase the trip point. As long as you don't crank for too long, it shouldn't overheat. Some circuits come with a thermostatic switch to kill output if it overheats.
 
fechter said:
You might be able to just add some shunt resistor across the existing ones to increase the trip point. As long as you don't crank for too long, it shouldn't overheat. Some circuits come with a thermostatic switch to kill output if it overheats.

Yea, that is nice suggestion I will use on my next battery with different type BMS I got but, linked above BMS seems to measure current based on FET resistance, no shunt resistors on it.
I could mes with sensing resistance to the chip but, chip has sanded down markings and can not identify it to get specs sheet.
 
Why would I need any BMS on 4S battery pack?
LVoltage buzzer is all that I need.
That what it SAVED my battery, many, many times!!!!
No cheap shoddy BMS.
Imagine current on car V6 engine start up battery, what BMS can handle it
 
miro13car said:
LVC off is all that I need
That what it SAVED my battery, many, many times!!!!
No cheap shoddy BMS.
Imagine current on car V6 engine start up battery, what BMS can handle it

How did you do that?
 
I have to correct myself here.
I dont have LVC
Sorry for confusion.
I have LVoltage buzzer from celllog.
So celllog shows me cells votages and sounds low voltage buzzer
4s battery is permanent connected in place of original lead acid battery.
Car charging curcuit charges only to max to 3.66V per cell
- very easy life for Nissan Leaf cells /Leaf cell charges max 4.2V/
After 4 winters voltage differences are the same between cells.
 
This is interesting. Conclusion is:

I need a low voltage cutoff. Either a stand-alone LVC, or a BMS which include LVC functionality.

Current wire harness have 3 cables:

- Positive - High amp cable straight to starter motor (60-70A peak during crank, 20A continious)
- Positive - Low amp cable through low amp fuse (7-10A) into rest of electrical system
- Negative - Common negative

Instead of LVC-protecting entire battery ($100+), I LVC protect the low amp circuit ($30+).
I want to cut the low amp wire in half, and put a low amp LVC ($30?) between? I have found LVC with both positive and negative, I would need if possible with only positive.
 
And use common sense when the batt is getting to low SoC, every 10-sec of cranking then is taking maybe dozens of life cycles off the back end.

If care is otherwise stellar anyway. . .
 
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