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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

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  • \$\begingroup\$ I am open to making my own charge controller if a schematic is available \$\endgroup\$ – Jack Wilson Apr 20 '17 at 5:03
  • \$\begingroup\$ Charging with constant current (one tenth of the capacity) is quite common. I did go for the same approach for the same purpose for years and still I'm doing. No problem I encountered. \$\endgroup\$ – Rohat Kılıç Apr 20 '17 at 5:16
  • \$\begingroup\$ Thanks, do you have experience with these kinds of batteries because I am not sure what voltage to use. Most people say 9V but I think it's worth a check \$\endgroup\$ – Jack Wilson Apr 20 '17 at 5:27
  • \$\begingroup\$ @JackWilson I don't think there is any online source which recommends what the OP is doing. Every source I have ever seen says to charge NiMH at constant current until -dV occurs or until the temperature rise increases dramatically. There are also some schemes based on timer only, such as C/10 for 12 hours, or C/3 for 5 hours (after first discharging). \$\endgroup\$ – mkeith Apr 20 '17 at 5:30
  • \$\begingroup\$ @JackWilson I don't know any specific method for determining the charging voltage but I set the charging voltage to 1.2 times the nominal battery voltage (i.e. 20% higher) and the current limit to C/10. For example, I'm charging 12V and 3.7V batteries with 14.4V and 4.5V, respectively. But make sure that the battery does not overheat. \$\endgroup\$ – Rohat Kılıç Apr 20 '17 at 5:42
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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.

enter image description here

More here - scroll, don't click.

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There are two reliable methods of charge termination with NiMH. In both cases, you charge at a constant current. The first is -dV/dt. In this method, the charger monitors the output voltage and watches for it to start to go down. When this happens, end of charge has occurred.

The second method of charge termination is dT/dt. The charger monitors the temperature of the cell, and when it starts to increase rapidly, end of charge has occurred. Obviously this requires some kind of temperature sensor.

In both cases, the charge rate needs to be reasonably high to produce a strong signal. Probably the ideal range is C/3 to C. For a dedicated charger, -dV/dt is probably easier to implement since you don't need a temperature sensor. If you plan to charge the batteries while there is a load attached, then you should use dT/dt.

There is a third method of charging NiMH batteries, but it is not ideal. Basically, you charge them at C/10 for 12 hours. This is the only method that can be used with a lab supply, and it only works if you remember to remove the batteries after 12 hours. The main downside to this method is that if the batteries are already charged when you connect them to the supply, then they will get an overcharge.

This is just an outline. If you decide to make a charger, you should read up on charge termination methods from a reliable source.

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It would work, but normally charging is stopped somewhere between 85 and 90 percent of capacity because retention after that is not as good, and the chance of outgassing increases.

The charge controller recognizes this by monitoring the voltage -- at 70%, the voltage begins to rise more quickly, and then plateaus off at 85%. Your bench power supply would try to charge the cells even more here.

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