Thursday, 24 January 2019

Generator charging ?

When talking to Richard the marine engineer the other week we were discussing charging batteries. His sage advice for older engines is to use a generator and decent multistage charger on the logic servicing generators and ultimately replacing them is a dam sight cheaper than the boat engine.

He did not comment on the respective fuel consumption but I'd guess a little genny is more frugal than a three litre boat engine.

So that bit makes sense - plus you don't get the same noise in the boat while the genny is running on the side  - just have to be aware of the neighbours I guess.

One think that is needed of course is a decent charger. I have one on the genny of course but I expect its low amps and is not clever multi stage to get the right volts into the battery at the right time. What a decent charger obviously does do is flex the volts for the best charge, unlike the basic charging system on the (my) boat.

That's all well and good but I do not have a charger on Percy. I have a 20 amp car charger but that does not seem to work off the genny and is basic. 

It gets more complicated then if I start thinking about putting one in the boat. As there are inverter charges or just charges. Now I have a 1kw inverter - quite old bit I rarely use it in favour of a little 125 w job that runs the tv and laptop charges as needed and I'm aware running an inverter at low demand to its capacity is inefficient.

Then there is the redundancy aspects - if an inverter charger dies you loose both at the same time and more expensive to replace. However I like the tidiness of an all in one unit.

So what do you have and why?


Andy Gic said...

We have the mastervolt 2500 inverter charger. (it was already aboard when we bought FL). After having a problem with the charging side and the whole unit having to go back to Holland for repair (which took six weeks) I am now of the opinion that separate units maybe the way to go if you have the room for them.

Nev Wells said...

Hi Andy,

Thanks for the comment. I think you have just cemented my thinking on combined units. Certainly if space is ok and budgets limited just the charger is the way to go IMHO.

Brian and Diana on NB Harnser said...

If you are using a genny you don't need a multi stage charger as once your batteries are charged you will turn the dam thing off, so it wont ever go into float. You just need a big basic one to get your batteries up to full charge so you can turn it off, unlike a shore supply where the volts are going to keep coming after the batteries are full.

Nev Wells said...

Hi Brian,

Thanks for the comment and your help on the 12v boating group. I’m going to wait till the engine is back operational and the BM712 is fitted, I’ll then have a good watch at what is going in to give an idea of what size I may need. I gather gennies are not your favourite thing ?


Tom and Jan said...

My suggestion would be to start with the size (or intended size) of your battery bank. Then establish how many amps (on average) you need to put back into your batteries. Remember to allow approximately 50% on top of that (you have to put more in than you took out!)

Let's assume you have a battery bank of 200ah and your average use is 100ah. You will need to put 150ah back into the batteries. Your recharging rate should be approximately not more than 30% of your total capacity. So the maximum recharging rate would be 60amps. The batteries will take up to 60amps until they are 80% charged (Bulk Stage) 80% of 150 is 120amps. The maximum charge rate is 60amps, so it will take 2 hours to Bulk charge the batteries. The remaining 20% charge is the Absorption Stage where you are attempting to squeeze the last of the charge into the batteries. This will take quite a long time and the batteries will only accept a low number of amps. Solar panels are very useful for the Absorption stage.

In the above example we needed to put 60amps back into the batteries for 2 hours.
Roughly convert that to Watts (V x A) 14.4V charging rate and 60 amps = 864 Watts. so a 1000W generator would probably Bulk recharge the batteries in two hours. After that you're burning petrol for not much recharge and would probably be better to use solar panels for the Absorption Stage.

We had a 450ah battery bank on Waiouru and when we weren't cruising sometimes used a 2000kva inverter generator to recharge the batteries. The generator would run for 2-3 hours on full power (Bulk Stage) and then you would hear the generator RPMs drop as the batteries went into the Absorption stage. The generator supplied 240V to a Victron 3KVA Inverter Charger.