.png)
1) What size should it be?
In systems where the AC charging is the 'last resort' it is usually only called upon when the batteries have become fairly heavily drained due to the solar being unable to keep up with the load usage (e.g. after a week of heavy rain or overcast weather). In this condition the battery most likely needs a heavy charge with as much current as possible. So the limiting factors on the charger's capacity will be:
- how big is the generator (or the maximum allowed draw from a grid-connection)? The charger can usually only be up to ~1.3x the kVA rating of the generator before the generator's output capacity becomes the limiting factor.
- what is the nominated maximum charge current for the battery? Regardless of any other factors, don't exceed this!
- what is the 'nominal' charge current of the battery? This is usualy a good size to aim for. Just be aware that while it's usually OK to use a smaller charger (in some cases a MUCH smaller charger), they will take longer to recharge the battery and if the battery has been damaged/over-discharged they may not be able to recover the battery from the depleted condition without a higher current capacity.
- don't forget to consider your cable and fuse sizings. These should be sized to handle the maximum charging/discharging current.
2) Can one charger look after multiple batteries?
Sometimes.
Most chargers will have only a single DC outlet for the battery bank.
Some models have dual DC outlets: one for a high current line to the large bank and a secondary trickle-current line that is usually used to recharge the generator's starter battery.
Other models may have multiple full-current outlets that can be used to charge multiple discrete battery strings.
3) What is the difference between a Battery Charger and a Rectifier?
Not much!
In most cases a battery charger will be a wall-mounted unit while a rectifier is usually a 19" rack format housing. They both take an AC source and use it to charge a DC battery system.
Battery Chargers can be anything from a small handheld unit to charge a small battery (e.g. a scooter) to a large bench/floor/wall mounted utilitarian box for delivering high current charge to a large off-grid solar system.
Rectifiers are most commonly used in UPS systems, so spend the majority of their time maintaining batteries in a FLOAT state. They usually do also have the ability to delivery BULK and ABSORB stage charge cycles, when they are recharging the UPS batteries after a power outage event.
4) Can multiple chargers be installed on one system?
Usually, yes.
Some Battery Chargers have some form of inter-device communications option. This allows them to 'synchronise' their charging behaviour in order to provide a consistent charge profile to the battery.
This isn't strictly a 'critical' feature (and there are many models that do not have it), but synchronised charging when using multiple devices is better for the long term performance of the battery.
5) What does 'Multi Stage' mean?
BULK: This is the first stage the charger will enter, and it assumes the battery is either heavily or somewhat discharged (i.e. in need of charging!) In this stage the charger will push as much current into the battery as it can. As the battery SOC increases it's voltage will gradually rise. Once the voltage recovers to the programmed level the charger will then hand over to the next stage.
ABSORB: As the battery nears full charge it's internal resistance increases, making it harder to charge and risking damage if the high current feed of the BULK stage is continued. In the ABSORB stage the charger will hold the battery at a preset voltage and gradually reduce the amount of current it delivers (so as not to stress the battery). This stage usually only lasts for 1-2hrs.
FLOAT: Once the ABSORB stage timer has completed, the battery is considered fully charged. The charger will now revert to a slightly lower voltage and just a small current - essentially only trickle charging - in order to maintain the fully charge condition.
A charger may run through several BULK/ABSORB/FLOAT rotations in a single day, particularly if there are heavy loads drawing power out of the battery throughout the day.
EQUALISE: This stage is only used for flooded lead-acid batteries. It is used once a month, or maybe as infrequently as once in three months. This cycle deliberately raises the battery voltage far higher than any of the other stages, though it usually keeps the current quite low. The cycle is a forceful method of 'equalizing' the voltages of all the individual battery cells within the bank. When used correctly it can significantly extend the life expectancy of flooded batteries, though additional maintenance activities are required to ensure this (not conducting the maintenance will result in early failure instead!).
This cycle may occasionally be used for sealed lead-acid batteries (referred to as a 'Conditioning Charge') as a manually initiated one-off recovery cycle in the event the battery had been deep discharged.
It will never be used for Lithium batteries.
MORE STAGES: Some charges offer additional stages beyond these standard four, such as specific profiles for recover of damaged or deep discharged batteries or profiles to maintain a battery in a 'storage' state (i.e. to counter the natural self-discharge of an inactive battery).