Saturday, 26 January 2019

Generator charging - those in the know give advice

Many times I have gained valuable insight into a subject pertaining to the boat - and my last post here and on the Facebook 12v group (closed, but easy to join and some very level headed and valuable advice given, with no judgement ) has delivered more sage advice on the issues around charging your batteries via a generator.

So with the gratitude to Phil who set up the 12v boating group, Brian off Harnser who has given me good advice in the past and Tom ex of Waiouru all the way from very hot Australia here is the advice. 

I have deliberately put it into one post so I can refer to it when eventually getting onto my mini electrical adjustments.... that I'll blog about of course. 

So Phil says....

The size charger will depend on the depth of charge you take the batteries to. The shallower you cycle them the less charge current they will take. 

In terms of alternator size for that bank the general rule is 20% of capacity so 60A however this will not charge at this rate for long unless batteries are quite low. In terms of a charger I would probably be looking for around 50A. 

You don't need to worry about great 'intelligence' on the charger, in fact you will be better off without any as these university educated chargers are really designed for long term landline operation and can easily undercharge in a sporadic mode of operation. 

You will be wanting to turn off charging as soon as possible so a float setting isn't even needed. Your battery monitor will be able to tell you when the batteries are full by monitoring the charge current and when less than about 1.5% capacity, 4.5A in your case, they are full. 

Don't ever rely on the monitor's battery percentage reading to stop charging as they are usually wrong and will often say full when they aren't. 

If you have AGM batteries then they can take a higher charge. You could use more than one charger if 2x30A are cheaper than 1x60A

Tom Says...

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.

...and Brian says 

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.

Thanks also to Andy who confirmed my thinking about splitting the charging and inverting  - his advice comes from experience,  the best sort of advice in my book.

So I know more and will now need to think more seriously about batteries  - I know I have waffled on about them a few times and I think I know what its between now but that'll wait. 

Also some news about the donk's head and the need for a couple of parts, nothing critical and understandable  - but thats for another post !

1 comment:

Dave Ward said...

My ten pence worth:

1) Tom's advice on calculating the power (VxA) is correct, but you also need to allow for the conversion efficiency (usually better than 80%, AND "power factor", which is a feature of AC circuits, but not often mentioned. Cheaper Switch Mode power supplies & chargers often have poor power factor correction, and this can markedly increase the load on any generator they are being run from. Before you buy anything, get hold of the FULL owners manual (you may have to go to the manufacturers site) and see what figures they quote. PF is shown as a decimal (such as 0.8), meaning it would take some 20% more power, and the lower the fraction, the worse things are. You need to multiply both losses, so a 50 amp charger (nominally drawing 700 watts, could easily put the equivalent of 1000 watts load on your genny.

2) Phil's comment about "Intelligent" chargers is good - I've found that some are quoted as having "Soft Start", which might sound a good idea, but really means they will only deliver a limited current until the battery voltage rises to (typically) 12.5 volts. Not what you want when trying to get a large battery bank re-charged quickly from a portable generator. Again, you should be able to find this out from an owners manual.

3) He is also correct about not needing the "Float" stage, but I haven't seen any chargers without it - it's just part of the software control anyway. If you don't leave the batteries on charge long enough for the current to drop right down the charger will stay at 14.5 volts (or whatever the "Absorption" setting is).

4) I don't know what sort of portable genny you are using, but look for a charger which has an adjustable current setting. Some of the better models have this option (it may be one of the setup parameters), and this will allow some trial and error to see what works best without overloading the genny. Better to have a reliable 50 amps than nothing at all!

5) I recently did some experimenting at home with my "Mains Fail" back up provisions, and managed to run 3 different chargers (totalling about 65 amps, or 910 watts) from an ancient Briggs & Stratton 1kW generator, but my mains power meter showed this lot were pulling around 1200 watts, and it wasn't very happy!