How best to charge your batteries

[RAB]

If moored for several days and no shoreline what's the best approach for charging the batteries?

1) Do you simply start the engine and leave it on "no throttle" idle?

2) Give it a few revs (assuming your not going to annoy the neighbours)?

3) One of the above but also put some load on, somehow (lights, or is fridge etc enough)? Not sure this is really necessary but seem to remember an AA chap telling me (after he had jumpstarted my car) to drive around for a bit with the lights on to help charge the batteries. Something about getting the "flux convertor" to kick in (I know it's not a flux convertor but I'm mechanical and can't remember the correct name!)

 

Second question - how long do you recommend running the engine for (I do realise it's a bit like asking how long is a piece of string!), but just a typical number based on your experiences will do.

 

Many thanks

[Ex- Member]

Solar panels.

[nicknorman]

Regarding the engine revs, it very much depends on the alternator / engine configuration - ie the ratio between the engine pulley, the alternator pulley, and the idle speed. Typically it is best to increase the engine revs a bit so with a modern engine idling at say 800rpm, maybe 1100 or 1200 is best to start with. However in order to find out what is best for your engine, you should apply a DC clampmeter such as a UNIT UT-203 (ebay £30) to the main alternator cable at the start of charging (batteries low) and monitor the charging current whilst adjusting the rpm. You will find an rpm where the best output is reached and above that, there will be no more increase. This is therefore the best setting for initial charging when the batteries can take the maximum current.

 

As the charge progresses the batteries can take less current so the rpm can be reduced, such that for the last hour or so the engine can probably be at idle or just over (again, you can check for the optimum rpm with the clampmeter).

 

You point 3) is a definite "No" - adding load will not achieve anything other than perhaps delaying the charge slightly.

 

How long to charge for? Yes, definitely a piece of string but if you tell us your battery bank size, your alternator size (or give us a clue by telling us the engine and its rough age) and how flat the batteries are when you start charging. But in general the answer is "longer than you think" to reach fully charged. If you don't reach fully charged from time to time, the batteries will become sulphated. Again, a UT-203 meter can help you because the closer the batteries get to fully charged, the more the current reduces. They are fully charged when the current has fallen to just an amp or three. Without a meter you are guessing in the dark.

 

In our case we have 450AH batteries and a 175A alternator (which is relatively big). 2-3 hours is sufficient to replenish most of the charge from say 60%, but would only reach about 85- 90% charged. The last bit will take another few hours. So I might charge it to 85-90% for a day or two but after several days I would want to get it to 100%. Because I have proper gauging it is easy for me to see how much I need to run the engine.

 

If you have a smaller alternator, bigger flatter batteries (ie more charge to replace) it will take longer.

Edited August 2, 2014 by nicknorman

[bottle]

What he said

 

Plus fit something to monitor your battery state of charge (SOC), a Smartgauge (cost in the region of £160) would be good, simple to fit, simple to read and you will know the SOC of your batteries.

 

ps,. I have no connection other than I have one fitted, to the boat not me. I know my batteries are sulphated, it's me age you know.