# Do battery cells in a series have the same C-rate?

If I wired up four NiMH battery cells from the same manufacturer in a series to power a load would the current and voltage from them continue to be split evenly or would it need an IC to monitor the charge/discharge rates of the battery cells to make sure they discharge and charge evenly so they all stay the same? Also would it need one monitoring IC per battery cell or can just one for all four work?

• ... to power a source What source? Did you mean load? Commented Jun 5, 2023 at 12:17
• @RohatKılıç Yeah, I meant load but said source instead by mistake. Commented Jun 5, 2023 at 12:58

Four matched NiMH batteries should share fairly nicely.

You can balance them by overcharging, as they tolerate indefinite overcharge as long as the current to overcharge them is not too high.

On discharge, you need to make sure that the first cell to become exhausted (there will always be the first one) does not get reverse charged (and so irreversibly damaged). On a long series string, this may mean monitoring every cell. On a 4S battery, you should be OK if you set your battery end voltage fairly conservatively, 4 V for 4 cells should be OK if they have been balanced during charging.

• By 4v do you mean the total voltage of all the cells would equal 4v for it to be safe or do you mean that I would use 4v to charge it? The 4 cell battery packs I see online are rated for 4.8v though I read that nimh batteries actually have more voltage than they are rated for at full charge, in this case 5.6v. Commented Jun 5, 2023 at 13:09
• I have a charging paragraph, and a discharging paragraph. But you are right, 'end voltage' is ambiguous as to whether it's at the end of charge, or end of discharge. As you correctly point out, the voltage at the end of charging will be of the order of 5.6 V, not that NiMH are charged to an 'end voltage' anyway. Commented Jun 5, 2023 at 13:21
• Just making sure i'm not misunderstanding, so you meant stop using the batteries when the voltage reaches around 4v during discharge and during charging I would need to make sure the cells are all balanced in charge and I would probably need 4 charging ICs to make sure each cell is balanced? Commented Jun 5, 2023 at 15:54
• When discharging, stop at 4 V. When charging, just whack a low current, C/10, across the whole 4S battery, and stop after 15 hours, the overcharge will automatically balance them. If you need a faster charge, then you need to terminate the fast charge on temperature rise to avoid damaging the cells. Commented Jun 5, 2023 at 16:31
• Ok, thanks for the information. Commented Jun 5, 2023 at 16:35

The voltage would split more or less equally, until towards the end of charge, when the voltage of the cell with the lowest capacity will drop out fastest.

The current is the same in each cell because they are in series. That's physics.

The c-rate is also the same in each cell because they are in series and because they have nominally the same capacity.

Ideally, yes, you would need a protector BMS (Battery Management System) (not quite an "IC", as you say) to stop discharge before the voltage of the lowest cell is reversed. However, the cost of such as BMS is so high that it is never implemented in low voltage NiMH batteries. (It is implemented in vehicles with NiMH traction batteries.) A battery with four NiMH cells is series is considered a low voltage battery. The hope (not the guarantee) is that the user will stop using the product when the battery voltage (not each cell voltage) gets too low, preventing damage to the lowest capacity cell.

Therefore, no, don't worry about it.

• So it should be fine as long as I make sure that the batteries aren't used too close to their minimum safe voltage (3.6v for all combined)? Would I need to just monitor the general voltage of all of the batteries combined to make sure they aren't getting too close to the limit? (or is this just unneeded?) Commented Jun 5, 2023 at 13:13