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Care for Ni-Cd & NiMH batteries

Care for Ni-Cd & NiMH batteries

Dieter, Jim

HOW TO

Trinity’s Jim Dieter explains it all

Of all the technical questions we get here at Radio Control CarAction, those concerning batteries seem to be the most frequently asked by RC newcomers and veterans alike. Seeking definitive answers, Q-and-A style, we turned to Jim Dieter-power broker to Trinity team drivers such as Brian Kinwald and Josh Cyrul.

CHARGING

Q: At what amperage should I charge my packs? Do packs of different capacities require different charge amperages?

A: As a rule, Ni-Cd batteries can be charged safely at twice their rated capacity; for example, a 2400mAh pack can be charged at 4800 milliamps, or 4.8 amps. Four amps is a safe rate for good all-around performance with any Ni-Cd battery. Higher amp rates can be used to deliver slightly more punch, but this is only effective if you run the pack immediately after you’ve charged it. Avoid amp rates in excess of three times the pack’s capacity, as they may lead to overheating of the pack. Unless you’re in a hurry, stick with 4 amps.

Nickel-metal-hydride (NiMH) cells should also be charged at 4 amps, but there is no benefit to charging them at higher amp rates. Unlike Ni-Cd packs, which “like” to be overcharged, NiMH cells don’t respond to multiple re-peaks or high-amp charging and may be damaged by such methods.

JIM DIETER:”The trick to getting maximum performance from any pack, whether NWH or Ni-Cd, is to run it immediately after charging while its voltage is highest After the pack has been removed from the charger, its voltage begins to flatten out.”

Q: My charger has adjustable voltage threshold. What should I set it for?

A: For Ni-Cds, it depends on your pack. Unmatched packs and older batteries that have a tendency to false peak may need a higher voltage threshold setting, while new matched packs can be charged with a lower setting. If your packs are coming off the charger closer to “hot” than to “warm,” try reducing the voltage threshold setting.

For NiMHs, you generally want the voltage threshold set as low as possible. If the charger continues to pump current into the battery as it waits for a deep voltage drop to indicate a peak charge, the pack can quickly overheat.

JD. “Even if your charger has adjustable voltage threshold, it’s a good idea to monitor NiMH packs as they charge. As soon as the pack’s temperature increases, you can consider it peaked.”

Q: My charger has “pulse” and “linear” settings. Which should I use?

A: Either setting is fine for Ni-Cd packs, but linear charging is generally preferable. Pulse charging can be helpful in getting a full charge into packs that like to false peak, but there’s no performance advantage.

Stick with linear charging for NiMH packs; pulse charging doesn’t seem to harm the cells, but the pack will tend to peak at a lower voltage than when using a linear charge mode.

JD: “NiMH batteries may be new, but they like good old-fashioned linear charging. If you’re shopping for a charger, pass up pulse- or flex-charging modes in favor of features you can really use, such as adjustable amperage and voltage threshold settings.”

DISCHARGING

Q: At what amperage should I discharge my packs?

A: Regardless of cell type, it’s best to discharge the pack at an amp rate that approximates what the pack encounters under race conditions; generally, 20 amps for stock and 30 amps for modified racing. If you’re using a bulb-type discharger with no. 1157 bulbs, use a 10-bulb discharger for a 20A load or a 15-bulb unit for a 30A load.

Q: How deeply should I discharge my packs?

A: At the very least, you should run your Ni-Cd packs until the car slows to a crawl. If you have a bulb-type discharger, dump the cells until the bulbs go out. For maximum discharging (and maximum cell capacity when you recharge), use a discharge tray like Trinity’s Real Time unit to individually discharge each cell. The Real Time 2 will bring each cell down to 0.5 volt per cell, or 3 volts for the entire pack. For most racing, this is as deeply as any pack needs to be discharged. If you don’t fully discharge your pack by at least running it in your car until it dumps, the pack will “remember” that it was recharged without being fully discharged. This will result in a very “flat” performance when it is discharged past the point where you have trained it to expect a recharge.

NiMH cells are not prone to “memory” problems like Ni-Cds, but it is still best to fully discharge them in the car or on bulbs, and then place them in a discharge tray. Don’t worry about dumping them below 0.9 volt per cell; there’s no benefit to extra-deep discharging.

Q: Will it hurt my packs if I run them more than once a day?

A: Recharging and discharging a Ni-Cd pack the same day won’t hurt it. The voltage is usually better on the second run, but there is a slight loss in run time. This is good news for stock racers; the increased voltage gives more punch, and the minor loss in run time isn’t an issue (unless you’ve geared the car to dump 10 seconds after the tone). If you continue to dump and recharge the pack, subsequent runs will be a little “flat,” but you won’t be damaging the cells. If you discharge and store the pack as this article explains, it will once again deliver maximum performance.

NiMH packs respond differently; feel free to run them two or three times a day, as the packs actually tend to improve with cycling.

JD: “No matter which type of pack you have, be sure to discharge it completely and let it cool before recharging it. A fan can help cool off your packs, but don’t use ice or a freezer, these can cool the pack too quickly, and the associated moisture may cause a short.”

STORAGE Q: Should I store my packs charged or discharged?

A: For best performance, store Ni-Cds fully discharged. If you store them charged, they will gradually bleed voltage until they’re depleted, and the pack will be very “flat” until you revive it with a few charge/discharge cycles.

On the other hand, NiMH packs should be stored charged. They don’t have to be fully charged; 40 percent is enough. You can store them discharged, but performance will suffer until the pack is cycled. JD: “Keep your packs in a cool, dry place when you’re not using them; a weekend in the trunk of your car in summertime is far more harmful to your packs than a full year in a box under your bed.”

Q: How long can I store my packs without damaging them or reducing their performance?

A: You can store Ni-Cds for a year or more with no damage, but you will want to cycle the pack for best performance when you’re ready to start running them again.

No one has an answer yet to the long-long-term NiMH-storage question, but it isn’t a problem to store packs for the duration of the off-season. Store the packs charged, cycle ’em before you race ’em, and they’ll be fine.

You may have heard rumors about NiMH packs “going bad” on the dealer’s shelf because they sat uncharged. It’s a myth, and there’s no need to worry about buying a “fresh” NiMH pack. All Trinity packs are sold with some voltage in them, and even if “dead,” a NiMH pack will perform at maximum capacity after a few charge/discharge cycles.

by the staff of RC Car Action & Jim Dieter

Copyright Air Age Publishing Summer 2002

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