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battery charging

If I install a new 240v battery charger on my narrow boat, do I need 1) a 2 bank charger to charge domestic and starter batteries separately,but at the same time, 2)some switching device to charge the batteries individually off a one bank charger, ie domestic or starter, or 3)can I connect all my batteries in parallel and charge them all at the same time. If connected together will my starter battery loose power when I'm running domestic only and will I risk running it down. Your guidance on the use of a one bank charger would be very much appreciated as internet advise on this although very good and informative, does not appear to address this item. regards Colin

Asked by: colin dixie  | 1.43pm, Wednesday 7 September

WW says:

Split charge diodes are not always useful with single charger outputs, as then the voltage sensing part of the charger will not work well, as the voltage of the starter battery is likely to be higher than the domestic, so when combined, a false reading can be obtained, which leads to poor charging of the domestic side. although split-charge diodes work well with alternators, using one with a single-output charger may well invalidate the guarantee.
Many dual output chargers have a fully battery sensing output, aimed for the domestic side, which give multi-stage charging regimes. As starter batteries usually just require maintenance charge (so a float level of 13.6V for most 12V lead-acid open cells) then the second output is often a fixed current, fixed voltage output. Some more expensive one do have a multistage output on the second (or third) output, which can be useful to allow the starter battery (assuming open cell) to reach gassing voltage of 14-6 to 14.8V periodically.
A very simple solution for the starter battery, is to buy a small, 3.8A or similar multistage charger (for around £15 to £30) and use this as a float charger for the main bank. It also gives you a reserve charger, should the main unit fail.
Just remember that the DC output cables should be fused and not just run straight to the battery.

Mark Langley  | 10.26AM, Friday 9 September

Also, I should point out that domestic and starter batteries are usually different in construction- so long-term paralleling of the banks is not a good idea- it is only ever for short term emergency starting, when the starter battery is flat/failed.

Mark Langley  | 10.37AM, Friday 9 September

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