I have 2 NiMh batteries 3500mAh 7.2V. However, first one have 7.58V(it increases each time Im measuring it) and second one have 8.37V.
Can this be the reason why 5x servos are doing unexpected movements and squeaky noises without even sending pulses to them(Im using arduino)?
Arduino Uno is powered by 9V battery and servos with that 7.2V battery.
Servos are HK15298 with volatge range 4.8~7.4V.
Are these 2 batteries already damaged and I should stop using them? Or how could I discharge them without damageing servos? To charge them I use charger with Delta Peak, but I dont think its working.
Best Answer
NimH cell voltage is not the best way to determine state of charge - but it can be used if other means are not available.
The normal means is to charge at about C/1 rate (eg 2500 mA for a 2500 mAh battery etc) and monitor the battery voltage for a DIP in voltage at full charge.
Another OK method is to charge at about C/1 and monitor the RATE of heating (not te temperature). When the heating rate suddenly increases the cel is ~= charged.
A 3rd method is absolute temperature - not recommended unless essential. If the battery gets too hot to hold it's charged :-).
Finally, cell voltage while charging can be used. This varies with charge rate and somewhat with battery model and manufacturer so you need to take due care.
NimH cells have a fully charged voltage ON the charger of about 1.45V per cell at lowish charge rates - say C/5 and below.
The "nominal" OPERATING voltage is 1.2V/cell so 7.2V battery contains 7.2V/1.2V = 6 cells.
6 cells fully charged on charger should be ABOUT
6 x 1.45V = 8.7V, so your measured voltage is about right at 8.35V.
8.35/6 = 1.4V / cell which is OK at very low charge rates.
7.58V/6 = 1.26V/cell so too low if on the charger.
Once charged and disconnected from the charger a fully carged NimH will read about 1.3V/cell.
1.3 x 6 - 7.8V
so IF the reported voltage if 7.58V is OFF the charger it is close to OK.
If it is while charging it is too low (probably).