Batteries

NiCad and NiMH Care

NiCad Care

You can discharge NiCad packs with a bulb discharger. However you must never allow the pack to fully discharge. If you do, you are taking a risk of reversing a cell. Disconnect the discharger when the lights begin to go dim. A better idea would be to use a cut-out switch to prevent the pack from discharging below a safe point. For a 6-cell pack a 5.4 volt switch would work, and for a 7-cell pack you would want a 6.3 volt switch. The idea here is to bring each cell down to .9 volts.

Another method of discharging your NiCad's is to run them in your truck until the point where it slows down dramatically on pavement, and just about stops in the grass. You will notice that when you pick the truck up, it still has enough power to turn the wheels easily. Just like with the bulb discharger, you don't want to run the battery all of the way down. Otherwise you'll take the chance of reversing a cell. At this point you should be around .9 volts a cell. It's not exact and perfect, but this method will work fine for the average backyard basher.

One of the more common mistakes I see is to take the pack at this point and slap it back on the charger. This is wrong, and will shorten the life of your pack. Make sure your pack cools completely before charging it again. You can use a fan to help move cooler air across the pack. However I discourage you from using the freezer, fridge, or any other extreme cooling procedure. While many people do this and they may not have any problems, it's not the best procedure for ensuring long life from your packs.

I also often see many people warn against using NiCad packs more than once or twice a day. I think this stems from the fact that you must ensure that your packs are completely cool before recharging them. I generally disregard this rule and feel that if you ensure your packs are cool, you should have no problems with running a pack several times in one day. I've got several packs that have been used in this manner for over a year and a half, several times a week, and have suffered no ill effects. In my opinion it's heat that causes battery failure, not how many times you run it in one day.

When charging NiCad batteries, you'll want the charger to see a higher voltage drop-off than NiMH would, when the pack is full. This is what the threshold setting on most chargers addresses. If you are using a charger that allows you to vary the threshold setting, you'll want it around 10-15mV for each cell. Therefore, for a 6-cell pack, you'll want to set it in the range of 60-90mV. The exact setting may vary between packs, and require some experimentation. Always start out on the low side and adjust upwards if you find you packs are not getting fully charged. When charging your NiCad pack, you should generally charge at a 4-5 amp rate.

Many high-end chargers will also provide you the capability of flex/pulse charging. This method of charging starts out at a fixed rate but at several points during the charge, it's interrupted with short bursts of discharging. This is often used to revive older batteries and restore much of their usefulness. This is used on NiCad packs, and isn't recommended for NiMH packs.

You should always make sure that your NiCad packs are discharged when they are stored. If you store a pack for a few days that's fine. However don't store the packs for a week, month, or any other prolonged period while they are charged. Doing so can affect the discharge rate and their capacity.

NiMH Care

The discharge procedure for NiMH packs is the same as for NiCad packs. Again, you want to discharge until you reach .9 volts a cell, but no lower than that so that you don't reverse a cell. As with the NiCad packs, you can also just run the truck until it slows.

Heat will kill a NiMH much easier than a NiCad battery. So you need to be especially careful that the pack is cool before recharging it. Again a fan is fine, but extreme measures can damage the pack. Also, you should always use a NiMH compatible peak charger when charging NiMH batteries. A NiMH battery pack has a much lower voltage drop-off when the pack is full. If you are using a charger that allows you to vary the threshold setting, you'll want it around 3-5mV for each cell. Therefore, for a 6-cell pack, you'll want to set it in the range of 18-30mV. The exact setting may vary between packs, and require some experimentation. Always start out on the low side and adjust upwards if you find you packs are not getting fully charged. When charging your NiMH pack, you should generally charge around a 4 amp rate. It is possible to charge at a 5 amp rate, but some NiMH packs may be less tolerant of the higher voltage. Flex/pulse charging isn't recommended on NiMH packs.

Probably one of the most significant differences in NiCad and NiMH packs is how to store them. When storing your NiMH packs, you should always charge them back up to around a third of their normal charge. Which, to say the least can be inconvenient. Who wants to sit around and charge their packs partially back up after they're done running. However it is necessary. NiMH packs can be damaged if you store them fully discharged. Contrary to popular belief NiMH can still suffer the same memory condition that NiCad's can. So when bringing your NiMH packs out of storage, it's a good idea to discharge them before you charge them. This will help them maintain their optimum level of performance. Running them down in your truck will suffice for this.

For more information on caring for your NiMH packs, see RCCA's article entitled NiMH Battery Care and Maintenance.

Repairing a Pack

There may come a time when a pack just doesn't seem to offer what it used to. One of the signs of a good pack is that it's seems to lose most of it's power all at once instead of throughout it's run. However if the pack seems flat soon after you start using it, or even from the beginning, it's possible you have a bad cell.

Just because a cell is bad doesn't mean you have to trash the whole pack. It's possible to replace just the problem cell and continue using the pack. Try to determine which cell is giving the problems. After a run, or after charging the pack, feel the individual cells and see if there's a drastic difference in temperature between any of them. If so, then that cell is a suspect. You can verify this when you break the pack open by checking the voltage on that cell to see if it is extremely lower than the others in the pack.

After determining the bad cell(s) you can replace them and rebuild the pack using the procedure found in the RCCA article entitled Build a Battery Pack. This article will walk you through the procedure of how to reassemble the pack once you have broke it apart. In addition you may also find the article entitled Rebuilding a Stick Pack of help as well.

Part One:	Introduction to Batteries
Part Two:	NiCad and NiMH Care
Part Three:	Using a Timed Charger