Battery Charger Query

[catalac08]

Hi
Looking at heavy duty battery chargers to quickly bring up battery banks by shore power or generator. What puzzles me is that different chargers specify say 50-120AH battery other 50-225AH batteries. I appreciate that the internal resistance of two batteries in parallel will be half that of one battery on its own and presumably the charging current will be more in total but I do not see why there is an upper limit on the battery capacity to be charged as bigger batteries would merely take longer to reach charge. Any ideas on this?

 

[claudio]

The charger should be rated at continuous duty if you wish to charge a higher battery capacity as the charger will be in bulk charge mode for a lot longer. If you double the rated capacity, it is likely that the charger will be operating flat out for twice as long. Whether your charger survives this depends on the design.

 

[VicS]

The chargers said to be suitable for the larger capacity batteries will have larger maximum current output capabilities.

This will enable them to charge the larger batteries, from a low state, in a more reasonable time than a charger with a lower maximum current output.

For example I have charger with a max output of 8amps. It is said to be suitable for batteries up to 100Ah. It'll charge bigger ones but it will take rather a long time if they are deeply discharged.

There is a 16 amp version of the same charger that is suitable for batteries up to 200Ah and a 4 amp version only really suitable for batteries of 50Ah or less.

For maintenance purposes, ie keeping a battery which is already charged topped up there are chargers with an output of 1 amp or less.

 

[mitiempo]

Flooded batteries will accept about 25% of their capacity in bulk charge - from 50% to 80% state of charge. If you have a 400 AH bank down to 50% that means a 50 amp charger will run flat out for almost 2 1/2 hours. A 25 amp charger will run twice as long for bulk and continue into absorption stage at max as well. A 100 amp charger will run just over 1 hour before current drops at the absorption stage. Ideally you want a charger rated at 25% of battery capacity for flooded, but larger is good also.

Better a large charger loafing along much of its life than a small one working at max most of the time.

 

[catalac08]

thanks claudio & VicS
what I have in mind is a 45 amp charger so that on my (infrequent) use of marinas I can charge up 4x110AH batteries overnight, but many chargers of this size still only rate them up to 225AH, without paying out silly money. I suppose like many things you get what you pay for!

 

[mitiempo]

A good quality 45 amp charger should work fine. With a good quality charger, after about 45 amps the price goes up considerably. Actually around here it is less expensive to buy an inverter/charger of 1800 watts/40 amp charge than a 60 amp charger from the same manufacturer (Xantrex).

Make sure it a a good quality 3 or 4 stage charger though.