Choosing the right battery pack charger

The importance of a good charging regime can not be overlooked when thinking about custom battery pack design. A lot of emphasis is put on the way in which the pack is discharged, capacity required, maximum operational and cut-off voltage, drain current characteristics, operational environment and temperature, run time to name but a few; the way in which these packs is charged is often put to the back of the design process with minimum research and development even though charging is extremely important in the general and long term operation of a given rechargeable battery pack.

There are a number of issues that need to be looked at when selecting the right charger for a given battery pack and these are broken down in the following paragraphs.

Charge Voltage

General Information

A general rule of thumb with regards to the charge voltage for NiCd and NiMh cells is 1.44V – 1.5V per 1.2V cell. This is the charge voltage used on most consumer chargers available on the market and is probably the most commonly quoted charge voltage value.

This is also a rule which can be transferred over to charging NiMh battery packs; say you have a 10 cell pack, nominal 12V output, you will need to use a charge voltage of at least 14.4V in order to charge the pack successfully. Again this is a standard nominal value used as a general rule of thumb and is not necessarily the optimum value for charging a given pack. In order to get a better understanding of charge voltages it is worth looking at the datasheets of individual cells and existing battery pack chargers.

What charge voltage is safe?

The question if often asked about what is a safe charge voltage when charging both cells and battery packs. An important factor to remember that is is the charge current rather than the charge voltage which is the most important factor to consider with regards to charging safety. This is not to say that voltage is not important and there are general guidelines to follow.

For fast charging: standard nominal voltage: 1.44V - 1.5V per cell; when 'Fast Charging' is mentioned, this refers to charging at a rate of 0.5C or higher, for example for a 2000mAh battery, charging at a rate of 1000mA or higher. When charging at this kind of rate, charge termination becomes very important and has to be very accurate to avoid damaging the cells being charged.

When thinking about standard charge rates of around 0.5C or less the nominal voltage level can be increased a little. Taking an example of a 10 cell battery pack, the nominal charge voltage for such a pack is 17.4V which equates to 1.74V per cell. This is a nominal voltage so it is possible to charge at a higher voltage, say 18V without causing damage, as long as charge termination is sufficient.

Effects on end of charge voltage

As with most electronic devices, the environmental conditions of operation have an effect on charge voltage of a cell. When thinking about standard charge rates in temperatures of 0°C to 45°C, as the temperature decreases the end of charge voltage increases. The following values are taken from the datasheet for the UH-AA2000G cell.
At 40°C the end of charge voltage is approx 1.5V
At 20°C the end of charge voltage is approx 1.57V
At 10°C the end of charge voltage is approx 1.67V
It can be seen from the values above that the temperature the battery is charged has an effect on the charge voltage selected and not taking this into account may mean that the connected batteries will not get fully charged.

Charge Current

The charge current delivered to the pack by the charger will determine the time it takes to charge up the battery, it is however important to avoid selecting the highest charge current available to achieve the fastest charging time. There is a limit to the charge current suitable for each pack, this is determined by the type and size of cell used in the battery pack, most importantly the capacity of the cells.
A value that is quite often referred to is the ‘C rate’; this value is used to breakdown different characteristics of different cells relative to their capacity. It is basically a fraction of the cells capacity so if you have a cell which is 2000mAh, 1C would be 2000mA, 0.2C would be 400mA; which can be applied to the discharge or charge current being taken out or put into the cell. When you look at the datasheets for the individual cells, various characteristics of the cells are given related to a specific C value. This may include the various charge rates, generally standard and fast, it is recommended to not charge at a higher rate than the specified ‘Fast Charge’ value; although it is possible to charge at higher rates, the cell manufacturer should be contacted first and additional safety features must be included in the charger to ensure safe charging.

Battery Type

Different battery technologies have different methods of charging so it is important that the charger is designed for the given technology. Nickel systems use the same charge method of constant current charging but this doesn’t mean that a NiCd charger can charge a NiMh battery or visa versa. This mainly applies to Intelligent chargers which detect the end of charge voltage, because the charge voltage characteristics of NiCd and NiMh are slightly different a specific intelligent charger for either of the two chemistries will not detect this end of charge voltage correctly and not charge the batteries correctly.

If your charger is not intelligent then it may be able to charge either of the types of battery, however you will need to look at the charging current and compare it against that stated on the datasheets of the cells you are charging. NiMh cells of the same size have a higher capacity than their NiCd equivalents so the same charger will take longer to charge the NiMh pack than NiCd.

Li-Ion has a different method of charging than Nickel systems and it is very important you use a Li-Ion charger to charge a Li-Ion pack. The charging process is critical with Li-Ion and it is highly recommended that you consult both the Li-Ion cell manufacturer and the manufacturer of the charger before developing any design using Li-Ion batteries.
Lead acid or Sealed Lead Acid also has a different charging regime to other technologies and so needs a specific Lead Acid charger to ensure correct charging.

Maximum Battery Pack Capacity

There are a number of factors that effect selecting a suitable battery pack charger as mentioned above, the voltage of the battery pack is one, as is the capacity of the battery pack.

Linked to the Charge Current section above, the capacity of the battery pack has a huge bearing on the charger you choose for your pack and there are a number of issues to take into account.

Basic chargers

These are chargers which act more like a power supply than a charger, simply putting in a constant charge current until it is turned off. For these chargers it is simply a case of looking at the charge current against the capacity of the cells used in the pack, making sure it fits with the charging spec on the cell datasheet and using the ‘Charge time formula’ shown below to calculate the appropriate charge time.

Intelligent or Timed chargers

This concerns chargers that are fitted with a safety timer which will halt or reduce the charge current to a trickle charge after a set time. This is a good safety feature to have on a charger and almost all intelligent chargers are fitted with them, as it means you do not have to worry about keeping track of the charge status of your pack and fear overcharging if the charge is left on for a long period of time. It does however limit the capacity of battery that can be charged and it is linked to the charge time table above. Using the formula below you can work out the maximum capacity the charger can successfully charge.

Electrical Circuitry around the pack

Battery packs are commonly fitted into some form of electronic equipment which has electrical circuitry connected to it. This circuitry can have a influence on the charging of the battery pack if it is seen by the charger. If you wish to use an intelligent charger then when you connect the charger to the battery, the charger should only see the battery; there should not be any circuitry between the battery and the charger. Additional circuitry which may contain diodes, resistors, capacitors etc. can affect the charge process and mean that the charger will not be able to detect the state of charge or even the type/size of the battery connected. This could mean that the charger may not be able to begin the charge process, not be able to detect the end of charge or apply the wrong charge voltage and current.

The thing to remember about this is to understand what king of charge control you want for your battery. If charge circuitry is to be built into the battery pack unit then you can’t use an intelligent charger, a power supply would be better for this kind of application. If you don’t want any charge circuitry fitted to the battery pack but you require charge control then an intelligent charger is the most suitable option.

Connection

A final issue to consider is how the charger is connected to the battery; there are many different connectors around with different devices having custom connectors developed for that product, other devices use standard type connectors. Chargers often come with some form of plug so the battery will need to have a matching socket fitted, these obviously need to match up but it is not just a matter of size, the polarity of the plug and socket are very important. Polarity needs to match otherwise there is a chance of connecting the battery the wrong way round with a possibility of short circuit. The picture below shows an example of polarity markings commonly seen on chargers and power supplies.