NIMH Battery Charging Question

[Flatshovel]

Hello,
I have a question. I have been reading some information that says that NIMH battery's should be fast charged rather than slowed charged and a full discharge should be performed once a month to reduce the memory effect. Now that brings me to my next question. I just recently purchased some new NIMH battery's for a pair of two-way radios that I have, unfortunately I only have slow chargers for them (10-14hrs). The battery's are rated at 1200mah and the chargers output is only showing 60ma. As long as I don't overcharge the battery's slow charging should be fine correct? Also would a full deep discharge say once every 30 days help keep the memory effect away? I would fast charge but I don't have a way of doing it and I am would rather not purchase 2 new fast chargers because of the price and I do not have a way of charging the 7.2 volt packs either. What are your thoughts on this? Just trying to maximize the life of the batts?

Thanks,
Joey

 

[qwertyydude]

Memory effect is almost unprovable and difficult to recreate even in lab settings, and is easily remedied with a few full cycles. Newer chemistries are said to not even have a memory effect and I'm apt to believe them. I don't know why people keep worrying about memory when the bigger worry is reverse charging due to deep discharging multicell packs.

It's almost ironic that people think "I need to deep discharge these batteries," but in multicell packs that is the worst thing you can do as the weak cells will reverse charge and get killed. The proper way to do deep discharge cycles is to do them with individual cells and not go below about .9 volts.

Just use them and when they start to get weak charge them, an individual channel charger with peak detection cutoff is better. Trickle charging is not ideal but it's better than reverse charging the cells which is why a majority of batteries fail and people blame "memory" when in fact the battery simply lost capacity, increased internal resistance and lost charge retention capability due to reverse charging abuse. Nicd's were more tolerant to reverse charging which is where the notion that killing rechargeable packs is ok. It's not with nimh. As much as it is tempting to just completely let it die with the intention you're preventing memory that's the worst thing you can do to your battery packs.

 

[Mr Happy]

I just recently purchased some new NIMH battery's for a pair of two-way radios that I have, unfortunately I only have slow chargers for them (10-14hrs). The battery's are rated at 1200mah and the chargers output is only showing 60ma. As long as I don't overcharge the battery's slow charging should be fine correct?

Charging the batteries this slowly really won't hurt them, so I would not worry about it. However, if your batteries really are 1200 mAh and the charger only outputs 60 mA, then the charging time is going to be long. A rough calculation is this:

charging time = 1200 mAh / 60 mA x 1.5 = 30 hours

This leads me to think that this might be a trickle charger that applies charging current to the batteries 24 hours/day. If so, that would be bad and will kill the batteries rather quicker than normal. To maximize the life of the batteries, you should put them on charge for 3 days until they are fully charged, then take them off charge and use them until they start to get weak before putting them back for another 3 day charge. But do not leave them on the charging cradle all the time.

 

[radellaf]

Memory effect is effectively not there for NiMH. Maybe if you let them sit a year and then charge, they may run at a lower voltage, but not if you're actively using them.

For a pack, a trickle top off is a very good idea as it helps make sure all the cells are full.

A couple hours C/10 or less won't harm modern NiMH.

There are dirt cheap chargers that will put out 500mA+, which would be better. But, yeah, eventually 60mA would do it.

 

[Flatshovel]

Thanks for all the replys. The radios in question are 2 Motorola MV21CV radios that I use on the 2 of the MURS channels. Now this brings me up to my next question. If I were to use say a 12vdc 500mah adapter plugged into the side of the radio where the connector goes to charge the battery's is the adapter in fact outputting 500mah? Some say yes to this and some say no. I would think that the place where you plug up the adapter on the side of the radio just connects the circuit together to charge the battery's and some how does not limit how much current comes from the adapter? Does that sound about right? If a 500mah adapter would work I think I might have a couple laying around in my junk drawer that I can wire up and use as I never throw anything away. I know this is not a two-way radio discussion forum but do you think the 500mah adapter Idea will work instead of using the slow 60mah ones?

Thanks,
Joey

 

[45/70]

Hi Flat. The problem with running an adapter as you suggest, is that you will likely have noise (on your transmitted signal) from the unfiltered power supply. That has been my experience with handheld HAM trancievers anyway. The only way they can have a clean signal using an external power source, is if you have a filtered PS, or are running from an external 12V battery, such as in your car. The problem with the later is, if the car is running, you may get alternator whine on your signal, unless adequate filtering is used.

As for charging NiMH packs, I've always preferred a slower charge, as opposed to a faster charge when charging NiMH cells in series. The slower rates when charging packs, help to reduce the chances of overcharging individual cells in a pack, where the cells are imbalanced, such as might be the case with older packs. My experience is mostly with tool packs, and over the years I noticed that the packs that were charged at an "overnight" rate lasted longer than ones charged with a 1-2 hour charger, such as I used at work. This was with NiCd packs, but I'm sure the effect would be even more pronounced with NiMH, as they are much less resilient to overcharge.

Dave

 

[Mr Happy]

OK, I found the user manual for that radio.

Apparently it takes a 3.6 V NiCd battery pack (not NiMH) and it uses a simple 10 hour trickle charging method (which is quite typical for NiCd). The manual says to put the radio on charge when needed and then unplug the charger when it has finished charging, and not to leave it on charge 24h/day (again, this is normal with trickle charging). The manual also says you shouldn't operate the radio with the charger plugged in as it might not work properly (as 45/70 suggested).

So, the answers to your questions seem to be:

1. Use the manufacturer's recommended battery pack
2. Use the charger adapter that came with the radios
3. Charge the batteries and use them according to the user manual

Now, you could use an NiMH pack instead of the recommended NiCd pack, but the capacity would probably be higher (2000 mAh instead of 1200 mAh). This means that charging would take proportionally longer (nearer 20 hours than 10 hours), but your run time would be longer when charged. Also, you would have to be especially careful to time the charge and unplug the charger when charged. It is very easy to ruin NiMH batteries by leaving them on charge 24h/day if you forget.

The snag with the kind of battery pack this radio has is that you will not find it easy to charge the battery outside the radio. You would have to rig up all sorts of special cradles, charging leads, expensive chargers and so on. It really would not be worth the bother.

 

[Flatshovel]

Thanks to all who reply-ed. The battery is 7.5volts instead of 3.6 and is rated for 1200mah instead of 600mah and is a NIMH battery instead of NICAD. I do not keep the power adapter plugged into the radio all the time and I do not transmit with it plugged in either. But my question is, If I were to use a 12vdc 500mah adapter instead of the 60mah adapter will it charge the battery at 500mah instead of 60mah? Just wondering on this.

Thanks,
Joey

 

[Mr Happy]

OK, I am rather puzzled about the details. The user manual for the MV21CV radio I looked up shows the battery as what looks like 3 cells shrink wrapped side by side into a pack with contacts on the top, and it says the pack is NiCd (the radio itself is a small hand-held unit, like a walkie-talkie). [Edit: I see it is a 7.2 V (7.5 V) battery.]

As to the question about using a different power adapter, the answer is no, don't try that. The radio and charger adapter were designed as a system. You need to use the charger supplied with the radio, with the battery designed for the radio.

Edit: This is what I found:

OK I found this:

Now it's clearer, but it still doesn't explain the NiMH part.

You could possibly use an NiMH battery in place of an original NiCd battery, but it would not be advisable.

 

[Flatshovel]

The example that you have listed is exactly the radio that I have. I bought the radio used and did not have a nicad battery with it, so I choose to purchase a new 1200mah NIMH battery to in that was available. I have been using it with no problems yet and seems to last well over 8 hrs which Is what I wanted. Now I know that NIMH prefers to be fast charged instead of slow charged, without being able to that would it be best to totally discharge the battery say once every 30 days or so to keep the crystalline formation down on it. I have read that nimh does require maintenance, however not as much as nicad and I have seen references to do maintenance to the battery once every 2-3 months is this correct? I presume that using the NIMH battery will be fine in the radio, just takes longer to fully charge correct?

Thanks,
Joey

 

[Mr Happy]

It's not quite true that NiMH prefers to be fast charged, it is more that systems of chargers and batteries can perform better when fast charging rates are selected.

In your case, I would try to keep the battery being cycled. Use the battery until it is almost empty (but not totally empty), then put the battery on charge and recharge it (but do not overcharge it). Since the battery has a higher capacity it will take 20 hours or more to charge instead of 10 hours (call it 24 hours). Whatever you do, do not leave the battery on charge indefinitely as that will damage it rather quickly.

 

[MikeAusC]

If you fast-charge Nickel batteries there is enough current to burn out Nickel whiskers that inevitably grow and increase self discharge.

 

[MikeAusC]

Memory-effect was where a battery that was frequently partly discharged eventually dropped in capacity.

But every Nickel battery will lose capacity if you store it and don't use it.

There's two options -
1. Store in discharged state
2. If they must be stored in charged state to be ready for instant use, then at least every 3 months you need to discharge each cell to 1.0 volts and recharge it.

18 volt tool batteries that I was about to throw away, now have excellent capacity, after following these options.

If your Nickel batteries have lost their capacity, 3 cycles of slow discharge and recharge will usually restore capacity.

 

[VidPro]

When i can get the pack out easily, Here we would take it out, and do a cycle everyonce in a while using Pulse charging or a hobby charger or bench supply or something.
so I get a bit of maintance on it, without much effort, it gets tested by default, and it can go back into its regular routine.

i have the 4X aaa pack motorolla and the slow charger, and the whole thing is pretty sad luckily it will also take normal AAs x4 after removing the pack.