How to: Look after NiMh Batteries

10 frequently asked questions

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Based on various magazine and website articles

  1. Which amp rate chould I use to charge the batteries?
  2. Which type of charger should I use?
  3. Can I use a mechanically timed charger to safely charge NiMH packs?
  4. My charger has adjustable voltage peak detection;
    which setting should I use?
  5. Which charge mode should I use?
  6. Will trickle-charging help or hurt my NiMH cells?
  7. Should I re-peak my pack before a race?
  8. How should I discharge a NiMH pack after a race?
  9. What about long-term storage?
  10. Can I run a pack of NiMH batteries more than once a day?

These answers have been obtained from various magazines and websites and should give you a good idea of how you should look after your NiMH cells to get the most from them. Team drivers will treat their batteries differently from a club driver as the team driver will get new batteries for each event and will not worry if they are ruined for the next event as long as they get the best out of them at the current event. The club driver wants his investment to last much longer so needs to avoid the extremes of charging rates somtimes talked about. A good charger with settings for NiMH batteries will always be a good investment.

1. Which amp rate should I use to charge the batteries?

Trinity team drivers have settled in on a 5A charge rate for them, although this is a little higher than the rate recommended for average users,it doesn't seem to harm the batteries. They have tested at higher rates (up to 9 amps!) but found higher rates unproductive. As with all rechargeable batteries, higher charge rates work the cells harder. The higher rates don't appear to have any positive effect on performance, so stick to the lower rates; 3 to 5 amps is best.
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2. Which type of charger should I use?

Almost any style of peak charger can be used to charge NiMH batteries, but caution must be exercised when using less sophisticated peak-detection chargers not specifically designed for NiMH batteries. Voltage peak chargers are designed to look for a certain voltage drop to indicate that a pack has reached maximum capacity. Ni-Cd batteries exhibit a greater voltage fluctuation during the charge cycle and a more noticeable voltage drop at the end of it. Chargers originally designed for charging Ni-Cd packs look for a comparatively pronounced voltage drop before they shut off the charger. NiMH batteries are more stable while being charged and demonstrate less voltage drop when full capacity has been reached. This makes it harder for a Ni-Cd-type battery charger to know when a NiMH pack has peaked. As a result, some of the less expensive, non-adjustable voltage peak chargers will over-charge a NiMH pack.
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3. Can I use a mechanically timed charger to safely charge NiMH packs?

Yes, but use caution. Continuously monitor the pack's voltage with a voltmeter to determine when it's approaching a full charge, and vigilantly check its temperature. As soon as the pack starts to get hot (this happens rather quickly as it reaches maximum charge), remove it from the charger. It's a good idea to monitor the pack's temperature even if you do have a peak charger; as noted above, the charger can overcook your pack if it is "looking for" the deeper voltage drop of a Ni-Cd pack to indicate a full charge.
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4. My charger has adjustable voltage peak detection; which setting should I use?

Chargers that have adjustable voltage peak detection should be set at 0.02 to 0.03 volt voltage drop (for the whole pack) before peak detection triggers and shuts off the charger. You can even go as low as 0.01 volt with good results, but such a low setting can lead to a "false peak."
You should also expect the NiMH batteries to be a little warmer than Ni-Cd cells when they've reached full capacity. When testing with a thermal peak-charger, Ni-Cds are typically at 35 to 37 degrees Celsius at full capacity, whereas the NiMH batteries are at 40 to 45 degrees C . Keep in mind that this is the temperature as detected by the thermal probe, not by your hands. The actual temperature of the battery is likely to be a little higher. Thermal peak charging is a very useful alternative to voltage peak detection, as it simply looks for a significant change in pack temperature. It's an excellent charging method, though it would be wise to activate the maximum charge time function in chargers so equipped to act as a backup in case the thermal probe is accidentally separated from the battery pack.
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5. Which charge mode should I use?

Many of the charge modes originally intended to increase the performance of Ni-Cd batteries are proving to be detrimental to NiMHs. Irrespective of the brand of charger, the results always appear to be better when they are charged in a linear mode, meaning there is no programmed fluctuation in amp rates and/or polarity. The special charge modes don't seem to negatively impact run time, but the pack's voltage is slightly lower. Bottom line: use linear charge modes when using a charger that offers that choice (it's the default mode for most high-end peak chargers and the only mode available on lower-priced units).
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6. Will trickle-charging help or hurt my NiMH cells?

Trickle-charging should be avoided when charging NiMH packs. Most chargers default into a trickle mode when fast charging has been completed. This mode was designed to keep Ni-Cd packs at their highest voltage until they were used, but it seems to have a "flattening" effect on some NiMH cells. If your charger's trickle-mode can't be turned off, just be sure to disconnect the pack from the charger as soon as it's peaked.
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7. Should I re-peak my pack before a race?

Yes. For maximum performance, have the pack "peak" minutes before a race, stuff it into your car and go racing. However, using this method leaves no margin of error for a false peak; a safer approach would be to start charging about 1 hour before a race. This will leave 40 to 45 minutes to charge the pack, and you'll have an extra 15 minutes or so to remedy a false peak, if you need to. If all is well, then you can simply re-peak the pack prior to placing your car on the track. The ideal is to avoid letting the pack sit in a state of full charge for longer than necessary.
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8. How should I discharge a NiMH pack after a race?

NiMH batteries seem to be far less sensitive to post-race treatment than Ni-Cds are. Trinity team guys discharge their packs by running them out in the car or placing them in a discharge tray. Their routine varies according to when a pack will next be used. If it will be used within a week, take it down to 0.9 volt per cell, or 5.4 volts in a 6-cell pack. The rate of discharge doesn't seem to impact pack performance. NiMH cells don't have the same memory characteristics as Ni-Cds, so it seems less important to follow strict discharge guidelines. By whichever means necessary, bring the pack down to 0.9 volt per cell, but no less. NiMH batteries don't respond well to being stored in a completely discharged state. If a pack is at a nearly zero discharge state and is put away for a week, it can easily bleed off the remaining charge and reach dead zero. If the pack is allowed to bottom out, it could be harmed.

If a tray is your chosen discharge method, the pack will have to be removed earlier than is typical with Ni-Cds. Remember, discharge trays designed for Ni-Cds will bring voltage well below what's considered safe for NiMH batteries.
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9. What about long-term storage?

If the batteries are to be stored for more than two weeks, it's wise to charge them to at least 50 percent of capacity. All rechargeable batteries lose their charge if left unused for long periods—a condition we already know isn't ideal for NiMH batteries. For lengthy storage, it's best that packs have a significant level of charge. A note for those who use battery "bugs," or cutoff devices: if they're designed for use with Ni-Cd batteries, they will discharge a NiMH pack to below a safe level. Seek out a device that's intended to be used with NiMH packs. With all of the information already, it should go without saying that you should never store packs with a dead-short lead.
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10. Can I run a pack of NiMH batteries more than once a day?

Yes; in fact, it's recommended. After a week of being stored, a pack's first run will tend to be "flat," and it will develop more punch during its second and third runs. You simply need to let the pack cool completely before you recharge it. Even the top team drivers will run the packs two and three times during a day.



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