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Old 28 April 2006, 17:18   #1
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Battery Switch

I have two batteries on the rib and a battery switch. One battery is fully charged, the other is almost flat (but otherwise ok).

Question is: Is it ok to start engine with good battery and then switch to both or just the flat one to charge it? I read somewhere that you shouldn't switch through both off whilst engine running, but is it ok to switch from one to two or to both whilst engine running?

Also any ideas how long a battery takes to recharge from an alternator (approx)

Many Thanks
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Old 28 April 2006, 17:28   #2
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It's not good to disconnect the battery whilst running.......
However, I can't see any harm in switching between batteries. Would take a good few hours to fully charge the battery on the engine; you'd be better to charge it with a mains charger.....
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Old 28 April 2006, 17:45   #3
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With the both off youíll destroy the alternator, i donít think you can switch while its running. Start on the low one with a jump start then use the boat and charge it. Donít switch it to both or you will have too low batteries. There are automatic switches for dual battery set ups i believe.
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Old 28 April 2006, 17:50   #4
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switch

I had a faulty battery switch on my rib once causing a fire in the engine, due to the charge not getting into the battery and loop charging itself stupid.
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Old 28 April 2006, 17:55   #5
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Quote:
With the both off youíll destroy the alternator, i donít think you can switch while its running.
When you switch between batteries there is a **split** second when the engine isn't connected to a battery. I dont think this will cause any damage.
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Old 28 April 2006, 18:03   #6
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wouldnt risk switching whilst its running-if the switch momentarily breaks before it makes the other set of contacts it can be expensive. My mechanic keeps telling me about the ones he has had to sort out after the owners have decided to give the second battery a boost on the way home. Having said that there will doubtless be loadsa folk who have had no problems with switching over-but why risk it? That set of jump leads sounds a good idea.
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Old 28 April 2006, 18:08   #7
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Start on the good battery and run it as normal for 15 - 20 minutes then switch through both to the flat battery then carry on running. Will take a couple of hours to fully charge it but should get to partly charged pretty quick.

By leaving it 20 minutes before changing, you replace the energy used to start the engine in the first place, leaving you with a fully charged battery just in case. Switching to both for 5 seconds will not harm the battery or the alternator. After all thats what its designed for.

Its worth finding out why the battery went flat in the first place.

Tim'mers.
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Old 28 April 2006, 18:34   #8
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Forget about charging it from the engine if you want the battery to charge properly. It won't.

The alternator will probably survive a split second disconnect with no problems (do it with engine idling an minimum revs though, where the output is lowest, I think it is the voltage "spike" that kills the regulator pack when it is charging away and all the load is suddenly taken off, rather than actually not being connected to a battery. Think of it as being like your prop suddenly coming out of the water on full throttle, you get the sudden shower of pistons and conrods and £ signs before your eyes!).

But the main problem is that if you charge a nearly flat battery from an alternator it will get maybe 70% charged in a few hours running, but will never reach full charge. Take it out and charge it on a mains charger, for best battery life you don't want a modern starting battery to be partly charged, they don't much like it. If it is a deep discharge it will be less fussy, but you should still take it out and charge it right up.

The same is true of any battery/alternator setup. You can run a battery partly charged in a vehicle and get away with it for a long time, but it will never perform as well as one that has been properly charged. No point having a backup battery in your boat/vehicle if it isn't 100%!
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Old 29 April 2006, 03:44   #9
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Switches like this are commonly used by WAFI's. They start the engine on battery 1, then switch to both for charging, and should run their "domestic" electrics from battery 2.

If the switch is designed correctly when switching 1-both-2 it should make the connection without breaking the connection. So moving from "1" to "both", battery 1 stays permanently connected. Then when you switch from "both" to "2" battery 2 stays permanently connected.

If you turn the switch in the opposite direction (ie. 1-off-2) then both batteries ARE temporarily disconnected from the alternator for a second or so and there is a risk of damage to the alternator.
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