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Charging a battery with low voltage

I experienced that the Bluerobotics charger would not charge my battery with the reason that the voltage was too low.

Since I needed the battery for the operation to continue, I had to find a solution.

My solution was to connect the “dead” battery to my second battery with the charging cable, for app 10 - 20 seconds. This was enough to raise the voltage enough for the charger to accept the battery and recharge it.

Is there a more correct way to charge a “dead” battery?


I have had this problem too with HobbyKing batteries, I thought there would be a low voltage cutout, but apparently not??
Using a HobbyKing battery and charger, to fix, I change the cell number on the charger to 3. Charge for a minute or so, then try going back to 4 cells. Not ideal I’m sure, but like you, not sure of how else to remedy.

Hi Mikxie,

@spotxuv is right, if you have battery that has been over discharged it is possible to bring the voltage back high enough that the charger will charge it by charging in a different mode for a short period. 3S or lead acid mode should both do the trick. This method is a lot safer and more controlled than your method, which won’t limit current and could potentially discharge or charge the batteries at too high a rate.

What caused the battery to be over discharged in this way? Was there an issue that you experienced? As noted in our documentation, you should avoid discharging our battery below 12V, and never below 10V.

The BlueROV2 does not have a low voltage cutoff, and voltage should be monitored to avoid over discharging. We generally call a battery discharged and pull the ROV out of the water once the voltage starts dipping below 13V. Note that these voltage guidelines are specific to our battery and may not be appropriate for other batteries, especially lithium-polymers batteries, which are more sensitive to over discharging.


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hey @adam

Should the battery voltage be 14.8V at max charge??

If so how much time of diving would it give from 14.8 to 12.0V?

Luis Gamez

Hi Adam,

Thank you for your reply, I might have run it below the recommended 10v and then failed to turn of the ROV after the dive. Trying the lead acid mode did not work. I am aware that connecting the two batteries is not the best way, so any other way would be welcome :slight_smile:

You can also try the NICAD or NIMH setting on the charger and set it to 1A and trickle charge until it gets up past the min lipo voltage. I’ve recovered LIPOs in the past this way. Just be sure to monitor it at all times. Never charge LIPOs unattended as that is when they are most dangerous, during charge.

LIPOs do not have a built in low voltage cutoff so you need to monitor them to not over discharge.

When you recover it be sure to use Balance Charge and check to see if all the cells come back properly. Sometimes draining a pack can result in a bad cell.

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Hi Mixkie,

To clarify, 12V is the minimum recommended voltage to discharge to battery to during use. 10V is the absolute bottom end discharge voltage of the cells, but discharging down to this level may significantly impact the lifespan of the pack. I would recommend keeping the pack above 12.0V at all times.

@luisgamez 14.8V is the nominal voltage of a 4S battery like ours, but it is fully charged at 16.8V. Discharging from 16.8V to 14.8V is roughly half the capacity of the pack, and 14.8V to 12.0V is roughly the other half. Run time is extremely dependent on the actual environmental conditions and flying style, but we find 3-4 hours total is roughly accurate for most realistic use cases.


Hi All

I just have the same issue, is giving the LOW VOLTAGE and is not charging.
Have read all and what will be the best way to recharge my battery, I am have both
products from Bluerobotics, the battery and the charger.
Waiting for best hint.

Hi Guys

Solved the issue tricked the charger to a different mode for a short time, and when back to Lithium Lipo 14.6V, and is charging.


I had a similar Issue where one of my stacks was down to 1.1V where all the other 3 were still at 3.3V+

I am guessing I have a bad batch of batteries (I built my own).

What I ended up doing it charging this one stack with my adjustable bench top power supply. (Connected on the 4S connector) Gradually increasing the voltage until they were at the same voltage as the rest.

I will be monitoring them but will most likely have to change them out.

I have a BR2 Battery that got down to 6.5 volts. Charger says Cell Error ( not Blue Robotics Charger). Can this battery be re-charged or is it DOA.

Hi Forum
I by mistake did over-discharge my BR 18Ah battery to 2V and it was not possible to charge it with the LIpo charger. I used a power supply and limited current to 1A and over a period of 5 hours charge it to 12V. And I monitored the balance between cells and no larger difference. After that I could charge battery with the LiPo charger without any problems. After that I have tested the capacity of the battery and it still have the original capacity of 18Ah

// Bo