# Properly Charging NiMH Batteries that Have been Stored for Too Long

For a Robotics Project, I've purchased two equal NiMH batteries, seven months ago.

• PRODUCT = NiMH Battery 12V 3000mAh XT30 Connector [MH-FC] 20A Fuse 12-20

There is no indication of how the cells are internally arranged, but there are 10 of them.

Each cell, when measured from the outside measures roughly 40mm height and 22mm diameter.

• Battery 1 measures 12.93 V with a multimeter. Battery 1 has been used a few times, in short intervals.
• Battery 2 measures 12.68 V with the same multimeter. Battery 2 has never been used, I just removed it from the box to take the measurement.

After searching, I've learned that it is recommended to charge NiMH batteries that are not in use roughly every 6 months due to their self-discharge.

Failing to do so can lead the battery to become too depleted, damaging permanently the battery. Most sources, here on SE, stated that 0.9 V per cell is the indication you should recharge, and 0.8 V per cell is the absolute minimum threshold before things become dangerous.

I understand that measuring voltages is not a completely reliable way to assess NiMH battery state of charge, due to the flat profile of the discharge curve, holding 1.2 V per cell for most of region, then sharply decreasing. But I don't see any other way I can do this.

My balanced charger, an IMAX B6 AC Dual Power Balancer, by SKYRC, says in the manual (page 6. v2.20),

"Never attempt to charge or discharge the following types of batteries ... A battery that is already fully charged or just slightly discharged."

• Question 1: Can I assume that these batteries are 10S, and do this practice: If battery voltage is below 0.9*10 = 9 V, then it is time to charge it.

• Question 2: The Web says charge it every 6 months, the manual says don't charge a slightly discharged battery. Which recommendation should I follow in this case?

these batteries are 10S safe to assume
If battery voltage is below 0.9*10 = 9 V, you don't know whether it's one cell at 0 V and 9 at 1.0 V. With "no load" voltage, I might go for 10 V instead (knowing the same argument applies).