Battery protection and management advice

Other Hardware General Electronics
1

Hi,
I am using for my Arduino project a 10Ah 14.8V LiPo battery, predicting an average 5-10 Amps usage with peaks of 20-30Amps of 1 second. But I have a couple of doubts about the electronics.

I am looking through the BMS boards , but am not convinced cause of the passive balancing (i.e. through dissipating energy off the more charged cells), since for my application battery life is very important.
I would like to dispense of that and just add a short circuit protection circuit with relay like this one together with an active cell balancer board (those I'v seen work well). Given my usage parameters is this a safe solution? (I am mainly worried about the cheap relay like this 10A@48V, do you have experience with those?)

Also, to read the battery capacity, can I simply place a voltage divider to scale the battery voltage range to the Arduino 5V analog In range or am I missing something?

Thank you for any thoughts, suggestions or shared experiences

2

If battery life is important LiPo may be the wrong choice - it demands very strict management and protection.
FiFePO4 chemisty is more forgiving and features an inherently longer life (several thousand cycles is commonly quoted). Its safer too. However capacity cannot be determined from voltage as the voltage regulation is better at a nominal 3.2V, and
charge-counting is needed for a good estimate of SoC.

Seen batteryuniversity.com ?

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I already bought a LiPo, of the types used in RC cars or drones, which I understand is among the best choices for lightweight, compact, and capacitative batteries (sorry my project is a walking robot btw).

4

I have some experience with using lipos with similar specs and use patterns.

I am unfamiliar with balancing during discharge (while in use). Balance charging is key, of course.

You can def and should monitor pack and/or cell voltages during use, a resistor voltage divider and analog input will work fine.

I am not going to watch a video in hopes that a schematic pops up on that relay short circuit thing. Neither, I suspect, will the ppl you really want to pass judgment on that.

Draw and post a schematic of it. Don’t worry about explaining it… the schematic will show everything they need to know, in quick time. That’s the point of schematics, dunno why they’ve fallen out of fashion.

Typically in my application area there is no short circuit protect, yes we hang it out over the edge there and yes, there is occasional, um, excitement.

Lipos need care and attention. Trust all the dos and do nots that you should educate yourself on if you haven’t.

But they are amazing when it comes to providing current when you need it.

Invest in the best lipo charger you can afford. A good one will not just balance charge, but provide adjustments and information about your battery. And provision for discharge and storage charging.

Buy good batts, treat them well, stop using any that aren’t capable of taking a balance charge and surviving use instances while remaining more or less balanced. A good pack will end up balanced.

You’ll get longer life charging to less than 4.2 volts per cell; I use 4.15 as a compromise, there isn’t much power in that first 0.05 or 0.1 volts anyway…

Aim for 3.7 volts per cell after recovery. This leaves some power on the table, but will greatly increase your battery life also.

A means for not going too deep in power grabs during high current phases of use could be good. In my application that is not done; typically alarms are raised to indicate that the potential for that exists: a low battery warning and a critical battery alarm. We heed them.

HTH and be very careful. Become an expert, it will save you grief.

a7

1 Like
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A good extra measure is this, plus handy to have anyway:

which I found googling “ lipo battery checker “.

They cheap - hang one on the balance cable during use. Or one per battery if you’ve multiples.

In addition to everything else. A second chance to avoid trouble.

a7

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The schematic for the short circuit protection circuit is not really important, but the relay through which is closed the battery circuit has to be able to sustain my 10A 30Apeak current, do you think such a Chinese relay can handle that?

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Sry, that is a very bad attitude. How can we judge its efficacy and appropriate ness otherwise?

a7

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I wouldn’t switch lipo with any relay on a bet. Use a Mosfet switch if you must switch.

a7

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All I was unsure of was the mentioned component with which I don't have experience so I thought I'd ask here, the circuit itself works, my bad it wasn't clear in the post, anyways... noted.

Could you clear up your mosfet suggestion? don't mosfets get really hot really quickly at these currents?

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I doubt it is appropriate. I woud be sure if I could see a schematic. I’m not going to do the work required to produce what you shoukd already have in front of you.

No, mosfets do not get hot with high current if designed in correctly and properly heat winked if needed. They are often used for switching massive currents, it’s practically their raison d’etre.

But I’ll leave that to the heavies to say more about.

Check out

for devices that might be of interest. They work well. I use them alla time, but TBH, I do not switch the lipos I have in high current deployment scenarios.

a7

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I'll check out that mosfet board, it does seem an interesting alternative to my initial idea, plus if you say its better then I may end up using it.

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I think you misunderstand.
Balancing circuits dissipate energy from the charger, if you're trying to over-charge a cell.
They don't use energy from the cell itself.
Leo..

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That's for charging the battery, but while discharging (i.e. while in use) they do (I think, pls correct me)

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I have never seen balancing during use.

The balance "port" is used for charging and discharging to achieve balance on the charger.

You can take a clue from the wire size.

There could be use of energy in some cells to help balance others while on the charger, but this would vastly complicate the circuitry, no one does it, so you can blame global warming on that inefficiency. :wink:

a7

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A simple BMS only protects the cell in the stack from too high voltage,
and that only can happen during charging.
Then the over-voltage protecting circuit dissipates the excess charging current into heat.
Leo..

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hmm ok. thanks

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You likely do want an over-discharge protection circuit in a multi-cell battery which monitors each individual cell and shuts down the system if any one drops to a critical level. The same function as for a single cell.

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