nimh batterys

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tyce

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is there a specific technique for charging nimh batterys on my vhf.
reason i ask is someone recently said that they should have special chargers and be slow charged, if this is the case why can you get charging leads that plug straight into the boats 12v supply.
i have just bought a set of new batterys and soldered them together as the last set were goosed so i want the maximum life out of them.
 
The battery lead for mine (XM2000) has a resistor built in and claims to take 7.2 volts. It's on board at the moment but from memory it's a 150ohm, probably 1/2 watt. Does the trick.
 
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should have special chargers and be slow charged

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What a strange comment! A characteristic of NiMh batteries is that they can be fast-charged much better than NiCd. However when fast-charging any battery it is vital that the amount of charge is known and cut off at an appropriate time. Ideally temperature of the cells should be monitored also.

Vic
 
No. The charge current must be limited to the desired value. The simplest way can be a simple series resistor. If your radio uses NiMh AA cells these will probably have a rated capacity of around 1200-1800mAh. To charge from fully discharged in, for example, 10 hours you need a current of around 150-200mA, or 300-400mA for 5hrs.

Vic
 
The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets. The cheapest way to charge a nickel metal hydride battery is to charge at C/10 (10% of the rated capacity per hour) or below. So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.

Using a timer it is possible to charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully discharged before charging.

If a temperature monitor is used NiMH batteries can be charged at rates up to 1C (in other words 100% of the battery capacity in amp-hours for 1.5 hours).

In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and .05 C. The voltage required for this is dependent on temperature, so be sure to regulate the current.

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That is all good information. You need to remember of course that the 12 volts in your boat is more like 14volts with engine charging.
This makes it very tricky to charge a 9.6 volt pack which will need about 11.2 volts to charge it. At 150 ma the required ressitor is 4 ohms for 12 volt supply but 14 ohms at 14 volts.
You need a current regulated supply which unfortunately can take several volts more than the output volltage to work correctly.
I think the simplest method is to use a resistor suited to 14 volt supply and accept that with engine stopped the current is going to be a lot less. ie about 30 hours to fully recharge. That will be very safe for continuous trickle charge. good luck olewill
 
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