Need help finding BMS for this lipo battery.

Thread Starter

-live wire-

Joined Dec 22, 2017
896
Hi,
I'm going to get this 4S 7AH lipo battery for an RC project. I need a BMS for it that prevents short circuits and overdischarge. All of the BMSs I can find have a cutoff voltage of 2.5V! This doesn't seem good, as isn't it supposed to cut off at 3V to be safe and maintain its capacity!? So where can I find a decent 4S BMS? Amazon would be ideal so it doesn't take forever to ship. I also only need around 70-100W from it max.
 

Hymie

Joined Mar 30, 2018
710
I suspect that BMS now have a cut-off at 2.5V (or lower) is due to improvements in lithium cell technology allowing discharge to this voltage without issue.

This unit from a UK based ebay seller has a spec cut-off at 2.7V with the option of adding a 10k ntc providing charge/discharge temperature protection.
https://www.ebay.co.uk/itm/3S-4S-5S-3-7V-Polymer-Lithium-Battery-PCB-BMS-Charge-Protection-Board-BalanceLAL/264502910469?_trkparms=aid=111001&algo=REC.SEED&ao=1&asc=20160727114228&meid=f1dbfe34620e4131bf210d0c99986f00&pid=100290&rk=1&rkt=4&sd=264502910469&itm=264502910469&pmt=1&noa=1&pg=2060778&_trksid=p2060778.c100290.m3507

or alternatively this unit has a similar spec cut-off voltage.

https://www.ebay.co.uk/itm/25A-BMS-3S-4S-5S-18650-Lithium-Battery-Protection-Circuit-Charging-Board-Module/312716234652?ssPageName=STRK:MEBIDX:IT&var=611434858201&_trksid=p2060353.m1438.l2649
 

Thread Starter

-live wire-

Joined Dec 22, 2017
896
I suspect that BMS now have a cut-off at 2.5V (or lower) is due to improvements in lithium cell technology allowing discharge to this voltage without issue.

This unit from a UK based ebay seller has a spec cut-off at 2.7V with the option of adding a 10k ntc providing charge/discharge temperature protection.
https://www.ebay.co.uk/itm/3S-4S-5S-3-7V-Polymer-Lithium-Battery-PCB-BMS-Charge-Protection-Board-BalanceLAL/264502910469?_trkparms=aid=111001&algo=REC.SEED&ao=1&asc=20160727114228&meid=f1dbfe34620e4131bf210d0c99986f00&pid=100290&rk=1&rkt=4&sd=264502910469&itm=264502910469&pmt=1&noa=1&pg=2060778&_trksid=p2060778.c100290.m3507

or alternatively this unit has a similar spec cut-off voltage.

https://www.ebay.co.uk/itm/25A-BMS-3S-4S-5S-18650-Lithium-Battery-Protection-Circuit-Charging-Board-Module/312716234652?ssPageName=STRK:MEBIDX:IT&var=611434858201&_trksid=p2060353.m1438.l2649
Ok, but all of these lipos I'm looking at buying say it's 3.2V minimum safe discharge voltage. I'm going to use a raspi for the project, so can I just have it monitor voltage and turn off a latching relay or something when it hits too low a voltage, monitored with a voltage divider and analog pin?
 

MrSoftware

Joined Oct 29, 2013
1,575
That's 3.2V per cell, so for a 4s that would be 12.8V cutoff. Look in the RC toy market, speed controllers cutting off at a pre-set battery voltage is common there. Maybe there is something off the shelf that will work for your case? Otherwise you don't need a full processor to monitor, just use a small voltage reference chip, an OpAmp to compare voltage and a mosfet to drive your relay, or just use a big mosfet instead of a relay. You can even put a POT in your OpAmp circuit to make the cut-off easily adjustable.

That said, there MUST be something off the shelf for this. Here is one, the voltage is a little too low for your use (maybe you can adapt it?), but an adjustable one of these must exist. If you can find one with a separate sense line, you can put a voltage divider on the sense line to make it adjustable. Read the specs to see how much current the sense line requires though, you want it to be itty bitty (micro amps) so it doesn't significantly affect your battery life.

https://czh-labs.com/czh-labs-low-voltage-disconnect-module-lvd-12v-10a-protect-prolong-battery-life-p0621.html?gclid=Cj0KCQiAiZPvBRDZARIsAORkq7f0X4qHjM6bMtf-iBYfkTFb7RtE3a8bMWzFx3kmqt4wn4X-M1llB28aAiiaEALw_wcB
 

Thread Starter

-live wire-

Joined Dec 22, 2017
896
That's 3.2V per cell, so for a 4s that would be 12.8V cutoff. Look in the RC toy market, speed controllers cutting off at a pre-set battery voltage is common there. Maybe there is something off the shelf that will work for your case? Otherwise you don't need a full processor to monitor, just use a small voltage reference chip, an OpAmp to compare voltage and a mosfet to drive your relay, or just use a big mosfet instead of a relay. You can even put a POT in your OpAmp circuit to make the cut-off easily adjustable.

That said, there MUST be something off the shelf for this. Here is one, the voltage is a little too low for your use (maybe you can adapt it?), but an adjustable one of these must exist. If you can find one with a separate sense line, you can put a voltage divider on the sense line to make it adjustable. Read the specs to see how much current the sense line requires though, you want it to be itty bitty (micro amps) so it doesn't significantly affect your battery life.

https://czh-labs.com/czh-labs-low-voltage-disconnect-module-lvd-12v-10a-protect-prolong-battery-life-p0621.html?gclid=Cj0KCQiAiZPvBRDZARIsAORkq7f0X4qHjM6bMtf-iBYfkTFb7RtE3a8bMWzFx3kmqt4wn4X-M1llB28aAiiaEALw_wcB
I am using a DC motor, so I am using a H bridge with no cutoff. I think I'll just use a latching relay and monitor voltage, with a fuse. But is something roughly this a safe and good circuit for that?
 

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MrSoftware

Joined Oct 29, 2013
1,575
1 - Check that the Raspberry Pi analog inputs can take 12v, I'm guessing the limit might be 5v or 3.3v, in which case you'll need a voltage divider, and should probably also add a zener to ground to protect the Pi inputs
2 - If you must build your own, I believe an analog solution as I mentioned above would be more reliable and safer, as you don't have to worry about keeping a processor running, plus an analog solution will likely use less power than a processor, extending your run time.
 

Thread Starter

-live wire-

Joined Dec 22, 2017
896
1 - Check that the Raspberry Pi analog inputs can take 12v, I'm guessing the limit might be 5v or 3.3v, in which case you'll need a voltage divider, and should probably also add a Zener to ground to protect the Pi inputs
2 - If you must build your own, I believe an analog solution as I mentioned above would be more reliable and safer, as you don't have to worry about keeping a processor running, plus an analog solution will likely use less power than a processor, extending your run time.
I was too lazy to put in the voltage regulator circuit I drew lol. I have a $2 board that works fine to convert up to 30V to any voltage a volt or two below the input. But it's great to hear that a solution like that would work. So if I know there is a max of 30A from the battery under extreme conditions, then is a 50A quick blow fuse fine? Is there the possibility of a partial short burning wires meant for up to 30A but not triggering the 50A fuse?

Also, the motor says it can draw up to 14A at 24V. But I am only running it from the battery which is 12-16.8V. So the current should be about 16.8V/24V*14A = 9.8A max according to ohms law. Then 19.6 total because there are two motors. But I have heard that motors are not linear. So is it possible it can draw more under certain conditions? Up to the 14A? Also, could it burn out by drawing too much current if the motor gets stuck? Do I need to watch out for that? Or would that be unlikely because the wheels would still turn even if it's not moving?
 
Choose your fuse size so the weakest link in the system won't get damaged before the fuse blows. We have no idea what motor you're using, or what any of the rest of your design is so it's not possible to answer your motor questions. Generally speaking, a spinning DC motor will generate back EMF, essentially the input resistance goes up. If the motor stalls, this goes away and you're left with only the DC resistance of the coils, which is usually relatively low, which means you'll get a lot more current flow, which can make heat, which can be bad for the motor.
 

Thread Starter

-live wire-

Joined Dec 22, 2017
896
Choose your fuse size so the weakest link in the system won't get damaged before the fuse blows. We have no idea what motor you're using, or what any of the rest of your design is so it's not possible to answer your motor questions. Generally speaking, a spinning DC motor will generate back EMF, essentially the input resistance goes up. If the motor stalls, this goes away and you're left with only the DC resistance of the coils, which is usually relatively low, which means you'll get a lot more current flow, which can make heat, which can be bad for the motor.
Ok, fuse thing makes sense. I'm using this not at all sketchy motor. But it does seem like it will be pretty powerful and should work well. So do I use a 15A slow blow fuse on each motor too? Or could that cause issues?
 

MrSoftware

Joined Oct 29, 2013
1,575
That's up to you and your application. If you're making a toy to be used by an adult and not overly concerned about part failures then maybe don't bother, but if you're making something that would be a risk to people or more expensive property if it failed in a bad way then that requires more thought. It will operate either way, but it's up to you to decide how much safety you think is appropriate.
 

Thread Starter

-live wire-

Joined Dec 22, 2017
896
That's up to you and your application. If you're making a toy to be used by an adult and not overly concerned about part failures then maybe don't bother, but if you're making something that would be a risk to people or more expensive property if it failed in a bad way then that requires more thought. It will operate either way, but it's up to you to decide how much safety you think is appropriate.
Ok. I'm just making it as a fun project for myself, so if it fails thats unfortunate but not a big deal.
 
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