# Alternators and Battery chargers

1. Aug 4, 2017

### Graeme McIntosh

If I had a mechanical rotating device rotating at 700 rpm connected to an alternator producing approximately 700watts; could I generate 2400watts if I were to attach 4 of them in series or parallel? Would that power be sufficient to recharge a 60kWh lithium battery that uses 27kW in 3 hours continuously?

Last edited by a moderator: Aug 4, 2017
2. Aug 4, 2017

### sophiecentaur

Do you mean that you have a load that requires 27kW for 3 hours continuous? (I will assume a yes)
So the load demand that you are talking about is 27X3 = 81kWh
Alternator will supply 700 X 4 = 2800W, doesn't it? (2.8kW)
How many hours at 2.8kW is needed to charge a 60kWh battery bank? (60/2.8=21.4 hours : almost a day) The Batteries will last for 60/27 = 2.22 hour
You then need a source of 27kW for the remaining time and you only have 2.8kW from the alternator.
So, if that's you actual problem, it won't work.
Perhaps you meant something different from what I have understood. Where have I lost you in the above set of musings?
PS Battery and alternator connections needs to be right for the required load voltage. If the load takes 12V then the alternators need to be in parallel (and synchronised!!) If they deliver DC then they will need to be regulated / matched fairly well.

3. Aug 4, 2017

### Graeme McIntosh

Yup. That's my problem. Clearly it won't work; but how do I generate the additional power with a maximum of 4 mechanical shafts is my next problem. I'm a little stumped....

I could use a smaller battery or use a more efficient alternator. I really can't use anything smaller than a 50kW battery.

They do have to be in parallel.

4. Aug 4, 2017

### sophiecentaur

A bigger battery would do the job but not once a day. A smaller battery would be no use at all because your alternators have such a limited capacity.
Bigger alternators are available. I think you will have to bite the bullet and spend some money.
Of course, if you want 12V.
I have seen 120A alternators on eBay!!
Is your prime mover powerful enough of four of those?

Last edited: Aug 4, 2017