I have a project that requires some battery power and I have a 7.2V 3300mAh NiMh battery pack. I would like to charge it from my lab bench power supply which is current limiting.
Would this be recommended?
I would prefer not to purchase a charger though if it means the pack will explode then well… Also could I have a recommendation on a good charger around the $50 mark.
This how I have been charging to so far:
Voltage: 8.1V
Current Limit: 300 ma
I leave it to charge until the current is almost 0
Would this charging scheme be recommend or safe?
Thanks Jack
By the way this is the battery I am using: https://www.jaycar.com.au/7-2v-ni-mh-3300mah-r-c-battery/p/SB2314
Best Answer
Short:
Charging a 7.2V 3300 mAh NimH pack at 300 mA max and 8.1V max should work well and be safe and non damaging at room temperature (say under 30 C).
Detail:
There is a fourth (at least) method of charging NimH cells.
It is about equivalent to what you are doing and I have had it work well in several hundred thousand solar lights.
You won't find this in as many references as the other methods.
You will find it implemented, often badly, in very low cost drill battery chargers and similar. The poor versions do not limit Vmax as sharply as required, leading to "cooking" of the cells and early death.
Essentially: Set Imax to some acceptable value and limit Vmax to a value where near full charge is reached but at which the cell will draw minimal or no current.
Depending on charge rate, cell temperature and model of cell used, there is a Vmax at which the cell will cease charging at around 100% capacity without indefinite trickle charge input current.
Charge rate will drop as Vchg approaches this voltage so time to full charge will be substantially longer than eg time to 80% charge.
At about C/10 or less a typical NimH AA cell has a Vmax of 1.45V. This depends somewhat on cell brand and model and using say 1.4V is safer at the cost of lower charge in some batteries. I arrived at this voltage by buying a few of every brand of AA NimH cell I could find and performing tests at temperatures ranging from about 20C up to 'far too hot'. While NimH upper charging temperatures should be limited (40C or less) you still do not want a system that "runs away" at higher temperatures. Ideally you want to prevent charging at too-high temperatures.
In the case of your 7.2V battery, as NimH is usually considered to have Vcell = 1.2V that is presumably an Vb/1.2 = 6 cell pack.
In that case a charge voltage of up to say 1.4V x 6 = 8.4V will be "safe".
So your proposed voltage of 8.1V errs slightly on the safe side of my recommendation as 8.1/6 = 1.35v/cell.
For interest: I used a slightly modified version of this scheme for charging NimH cells in portable solar lights. The usually used schemes were unsuitable as
- delta V is utterly swamped by changes in insolation (sunshine level) due to clouds, other shadows and device movement.
Thermal effects are swamped by solar heating (both Tabs_max and delta T).
Fixed period charging is defeated by changing insolation levels during charging and on a given day.
Of these the most suitable is probably timed charging with current logging but even that is affected by the exigencies of thermal variations and the fact that n minutes at m milliamps my not produce the same result as say m minutes at n milliamps.
Using Vcell-Max worked well enough to be usable.
In practice I added a system that altered the reference point depending on charge rate. I provided temperature compensation circuitry for another client but I don't think it was adopted.
Note that modern NimH cells over about 1800 mAh/cell at AA size MUST not be trickle charged once charging is complete. Older cells with lower capacity included gas recombining chemicals and mechanism to absorb H2 and O2 from overcharge electrolysis but this was omitted as cell capacity grew. Modern cells will die early from dry out if trickle charged. ________________________________________
"4th method" NimH charging in variable temperature, variable current application where other methods are unable to be used. It works.
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