Using Two Batteries

[CaptainBob]

How do you make the most of two batteries exactly?

If I've used battery 1 as my "domestic" battery and got it pretty flat, I can then use battery 2 to start the engine... but once started, I've been advised not to switch the battery selector switch for fear of blowing circuits... but then I can't charge battery 1 back up.

I guess it's OK to switch to running with both after the engine is started in this scenario?

Cheers!

 

[jwilson]

If you have a rotary OFF-1-2-BOTH switch it's usually OK to go from 1 or 2 to BOTH as long as you don't go through OFF on the way.

Better nowadays to wire in a voltage sensitive relay so the egine battery does nothing but start the engine, and once the engine is running the relay automatically feeds power also to the services battery.

 

[VicS]

With the sort off set up you have probably the best thing to do is to designate one battery as the engine start battery and the other as the domestic services battery.

Switch to the engine start battery and start the engine, When the engine has been running for long enough to recharge that battery switch to both on then to the domestics battery. It will help you decide when the first is charged if you have a voltmeter or an ammeter.

If the engine is running it is vital that you switch via "Both" if you have a rotary selector switch. If you have separate isolator switches then switch the second on before switching the first off. At no time switch both off when the engine is running as a voltage surge will more than likely blow the diodes in the alternator.

When switching from one battery to another while the engine is not running it is not important but many will favour switching via both off.

As suggested a voltage sensitive relay (VSR) will automatically switch in the second battery for charging when the first is charged A diode splitter will charge both together but the diodes impose about a 0.6 volt drop so the batteries are not then fully charged unless the alternator is converted to battery sensing or otherwise modified to produce an extra 0.6volts.
The device marketed by Driftgate as the X-Split is an electronic alternative to a VSR

To make best use of any form of split charging system it is probably best to separate domestic and engine electrical circuits. Making one battery dedicated to starting and the other to domestics, replacing a rotary selector switch if that's what you have with separate isolators, but adding an interconnecting switch so that if the need arises you can parallel both batteries.

 

[CPD]

Hi Bob, the others give good advice. From experience, I have ditched the rotary 1/2/both/off switch and have 2 heavy duty battery isolators. I purposefully positioned them in an awkward spot and labelled them in a non-obvious way in order to really make one think before moving them. As the others say, the vital bit is to have the alternator under load whilst the engine is running ie at least one of the batteries switched to "on". the reason for ditching the 1/2/both/off switch was that it failed and when I took it off and opened it uup, I was really surprised at how cheaply made it appeared. With a bit of thought, the 2 battery isolators can be made to do exactly what the 1/2/both/off rotary selector does, and it is one less bit of kit to go wrong. Good luck with whichever way you go !
Alan

 

[VicS]

Yep meant to add that 1,2,off, both switches are notoriously unreliable although there is no reason why a decently made one should be. Avoid cheap ones I guess.

You say "the vital bit is to have the alternator under load whilst the engine is running" Strictly that is not quite correct. If you disconnect the output wiring before stating the engine no harm will be done.
What you must not do is disconnect the load while the engine is running.
All the electrons rushing along the wire suddenly have nowhere to go and all pile up creating a voltage surge. A bit like what would happen if a barrier suddenly appeared on a motorway. All the cars speeding along would all crash into each other! Sorry bad analogy perhaps.

 

[William_H]

In any system using the 1,2,both switch it is important that you do not always start and run on both. This is because you may not sense the demise of one battery. Because the other battery or the 2 combined can still start the engine. You should at least regularly ensure that either battery alone can start the engine. That will maximise your chances of a start if the engine battery dies as it will of old age and the service battery is likewise tired or run down.
olewill

 

[absit_omen]

Good point Will.

I had been starting and running on both and could not unerstand why my voltage reading was getting lower and lower. After checking the individual voltages of all three batteries I found that the two domestic were fine but the engine start battery was down to under 8v. The domestic batteries hd been suporting the knackered engine battery,

Incidentally, the start battery had one of these 'magic eye' thingies which was still showing green - fully charged!

 

[Severnman]

Disagree Will. The requirements for an engine start battery and a general service battery are different. The former requires a large current for a brief time ie the most important criteria is its cold crank capability. Service batteries for lights, nav and radio etc will generally have a larger capacity but not the cold crank capability (for continual use). The construction is slightly different. It's still a good idea to be able to parallel both batteries in the event of a battery failure. To be honest regular battery monitoring, checking and maintenance should preclude this.

 

[CaptainBob]

Nice analagy /forums/images/graemlins/smile.gif

Cheers for all the replies - I think I get the idea.

 

[William_H]

Yes I understand the design for start batteries.They can provide avery large starting current for their size and cost. However most service type batteries are very big and so while not designed for max CCA are still very capable of providing starter current without damage.
If a service battery can not start the engine it is no good counting on it as an emergency start battery.
Many battery failure modes are a failure to provide high current while they may appear to have plenty of capacity for service type currents.

This question of how much current a service type deep cycle battery can supply is a variable quantity but presumably we need the service battery to be able to start the engine in an emergency. Obviously a tiny SLA less than 40AH may be damaged by high discharge current but a bigger one should be OK. We would hope. olewill