Combine or Isolate flat battery for emergency start?

[steve-ribnet]

Hi all,

In a 2-battery setup (one 'start' battery, one 'house' battery),

if the 'start' battery is too flat to crank the engine, and you want to use the 'house' battery to start the engine, is it best to:

a) combine (parallel) up the two batteries
or
b) isolate the flat 'start' battery and use only the 'house' battery to start the engine?

I'm thinking that :

combining one flat(ish) battery and one full battery may offer more current for cranking (the combined battery internal resistance is lower when paralleled) - so (a) is better,

but...

the combined battery voltage will be lower, so the starter doesn't spin as fast...so (b) is better...?


(I'm imagining the scenario where an engine problem has led to excessive cranking and flattening of the 'start' battery, as opposed to a problem with the 'start' battery itself causing it to be flat)


Is there an actual answer, or is it more of a 'depends on how flat...' kind of answer?

cheers


thanks

Steve

[jyasaki]

Best is to try the single charged battery first. When you switch to Both, the first thing that happens is that the good battery tries to charge the dead one. No sense pulling it down trying both.

If the charged battery won't start it, try both. As you said, you'll have less voltage, but more current capability for a given voltage. Plus, at that point, you have nothing to lose.

jky

[Erin]

Most modern split charge units will only allow you to parallel the batteries not swap the house for the domestic. I would therefore suspect that paralleling is acceptable. Internal resistance of the dead battery will stop it taking too much from the good one.

[jyasaki]

Battery 1/Both/Battery 2/Off is what most switches have. Depending on how you wire it up, (and the simple way is direct: select either or both), switching from 1 to 2 does exactly that.

The disadvantage of this setup is that you only charge the selected battery while running. I know a few people who have run into problems with that.

If you have a setup where you run a VSR and have a more complex wiring scheme, you may be right, and may not be able to swap one for the other.

jky

[Erin]

Originally Posted by jyasaki

If you have a setup where you run a VSR and have a more complex wiring scheme, you may be right, and may not be able to swap one for the other.

jky

I was referring to a VSR system. A 1,2 or both switch requires manual intervention and is fine for twin engine twin battery installations but not so versatile for single engine twin battery setups where your 2nd battery is for domestic use but still needs charging along with batt 1.

[steve-ribnet]

Interesting viewpoints, thanks all

The setup I will have is with a VSR (Voltage Sensitive Relay) as, although there is increased wiring complexity, it offers resilience against forgetting to switch a manual switch to the right setting.

As Erin points out, the VSR has a manual override allowing you to combine (parallel) the two batteries for 'boost start' as it calls it. On Gin Palace hardboats the house bank is usually considerably larger than the start bank so the drain into the start bank when paralleled is probably negligible.

With some though to the layout of isolators, you can just open the 'start' battery isolator whilst pressing the 'boost start' button on the VSR/manually combine the batteries ,and you've effectively got a 1/2/both switch but with automatic combining for everyday use.