Charging Batteries

 What types of NiMH battery charger are available?

There are essentially three types. In order of price and sophistication these are; manual slow battery chargers, IC timer controlled "fast" chargers and microprocessor controlled "ultra-fast" intelligent chargers (IC just stands for Integrated Circuit, meaning that the timing is done by a silicon chip).

Manual battery chargers gently charge your batteries with a small current. They are very slow (up to 36 hours for 2000mAh NiMH batteries) and the user has to manually cease charging by unplugging. This involves a bit of guesswork regarding charging times. Nevertheless, they are cheap and effective.

IC timer controlled "fast" battery chargers charge rapidly for a fixed period and then automatically switch to trickle charge mode. They charge in less than half the time of manual chargers, and you don't have to worry about unplugging them. However, a timer controlled battery charger can't detect the initial state of your batteries - if they are already fully or partially charged it will still attempt to charge them for the full duration of its timer, so they are not suitable for top-up charging. Nevertheless, high quality IC timer controlled chargers incorporate sensors which cease fast charging if an overcharge state occurs (this is termed 'overcharge protection').

Microprocessor controlled "ultra fast" battery chargers are the latest type and are completely automated. They continuously monitor either the voltage or temperature of your batteries and are able to determine precisely when to cease fast charging. They then switch to trickle charge mode. Batteries in any charge state can be brought up to full charge in less then about 3 hours without risk of overcharging. The best battery chargers of this type independently monitor and charge each battery (this is termed 'individual supervision').

 Which battery charger do I need?

A manual slow charger is cheap and effective, but charging times are long and will always involve a bit of guesswork. If you are not worried about your NiMH batteries being at their optimum charge level, this type will be fine.

A timer controlled fast charger provides some level of automation. But they are not suitable for top-up charging because the timer will attempt to run for its standard duration regardless of how much the batteries need charging. Nevertheless, many users are content to use their NiMH batteries until they fail and change to a new set whilst recharging the old set. Top-up charging is only necessary if you want to be assured that your NiMH batteries are always at peak capacity. So a high quality timer controlled charger with overcharge protection is suitable for most purposes - and they are excellent value for money.

If you want to be confident that your NiMH batteries are at maximum capacity a microprocessor controlled charger featuring delta V or delta T control is the solution. The process is completely automated and batteries can be topped up at will. This is the only charger type that ensures that the rechargeable batteries you are putting in your device are at their maximum energy level. Ultimately it all comes down to price!

  I already own a manual NiMH battery charger - will it be OK for 2000mAh rechargeable batteries?

It will be OK, but you will need to adjust your charging times for 2000mAh NiMH rechargeable batteries by adding about 10% onto the usual charging time for 1800mAh batteries.

 How long should I charge NiMH rechargeable batteries in my manual battery charger?

Always follow the guidelines provided with your charger if they are adequate. Failing this, a basic estimate for a manual charger can be obtained by dividing the mAh rating of your batteries by the charging current. e.g. 1600 mAh batteries charged at 160 mA should require about 10 hours. Many experts recommend the addition of extra time to this (up to 40%) to make up for inefficiency in the charging process; but all of these estimates are based on your batteries being charged from a totally discharged state. So we recommend you stick with the basic estimate, as it is always preferable to undercharge NiMH batteries than overcharge them. The alternative is to invest in an automatic battery charger.

Note: unless your charger specifically permits it, don't charge batteries of different capacities together in a basic charger as this can lead to overcharging of the smaller capacity batteries.

 What is meant by "delta V" control?

Delta V control is a sophisticated method of automatically detecting when a battery is fully charged. By continuously monitoring the voltage of the batteries as they charge, delta V controlled chargers are able to detect precisely when to cease fast charging and switch to trickle-charge mode. Because of their sophistication, delta V (and delta T) controlled chargers are able to "top-up" partially discharged batteries as required without risk of overcharging. Requires full microprocessor control (also see our NiMH Technical page).

 What is meant by "delta T" control?

An alternative to delta V control. When fully charged, NiMH batteries start to warm up more rapidly. Chargers which use delta T control monitor battery temperature and determine when to cease charging based upon the profile of these temperature changes. Requires full microprocessor control.

 My NiMH batteries get warm in the charger - is this OK?

It is normal for NiMH batteries to get warm during charging.

 What is "trickle charging"?

Charging at a very slow rate to keep fully charged batteries at their maximum energy level. Trickle currents are generally judged to be between 1/30 and 1/20 of the battery's capacity in mAh. e.g. for a 1600 mAh battery, suitable trickle currents are judged to be between 50 and 80 mA. IC timer controlled and microprocessor controlled chargers automatically switch to trickle charge on completion of their fast charge - this means you can safely leave them plugged in.

 My 'intelligent battery charger' tries to re-charge fully charged batteries - why is this?