Logic problem

What's the question? Where's the mystery?

Can you add a diode?

Mike

You need isolation diodes to prevent one battery from trying to charge the other AND to prevent back voltage to charger

Condition 1 - Batteries charging SPDT set toggled one way, so you have C, NC, and NO connections.

Battery 1 - Negative to circuit Common/Ground, positive to SPDT NO contact and also tied to charger via isolation diode #1.

Battery 2 - Negative to SPDT switch Common contact, positive has a connection to charger through isolation diode #2, with second connection on positive to load positive.

SPDT switch - NC - to Common/Ground

Load - Negative to Common/Ground. Positive goes to positive lug of Battery 2.

Charger - Negative to Common/Ground, (Positive has two isolation diodes previously mentioned that go to the positive terminal of each battery)

When under load SPDT switch is toggled other way, Battery 2 is then in series with Battery 1.

Do you need a sketch? Really?

John :-#)#

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Opps, misread criteria, that is a DPDT relay. So the second terminals SPDT contacts can isolate the charger if desired/needed.

John ;-#)#

Thanks, a picture always helps.

George H.

Does it have to be a relay? DC switching can be pretty tough on relay contacts.

Can you afford a few milliseconds in between configurations?

If so, I'd like to suggest you do this with "back-to-back" FET switches. Drive the gates with an photovoltaic output opto-isolator. (You'll thank me later..) :)

Does a lead acid battery have enough voltage output to charge another lead acid with enough currrent to cause a problem?

Do lead acids need a diode between charger and battery?

With such a diode in place, will the charger deliver enough voltage to charge properly? Will it even start charging due to not sensing 10v+ on the battery?

NT

Not sure what you mean by the above sentance. If you are asking if batteries can be put in series, well, yes, all batteries are are single

2.2VDC cells in series. Six of them make up a standard 12VDC battery.

If instead you are asking if you can charge one battery from a second battery, the answer is 'sort of'. It all depends on the batteries and their condition. You won't be able to fully charge one battery by simply hooking up a similar capacity and quality fully charged battery across the less charged one as the former will only charge the latter battery until both are more or less equal in voltage and current capacity (if both are of equal quality).

Only if you are charging two from the same charger at the same time. This will drop the charging voltage slightly at the battery side so is not ideal unless you can compensate the voltage drop at the charger.

Depends on the charger. More advanced ones read the battery, but unless they are designed to handle two batteries at the same time, they can't monitor correctly two separate batteries simultaneously. Batteries do not charge at exactly the same rate. I was considering a more basic charger that simply was voltage limited (and current) to roughly

14.5VDC, giving you a float of around 14VDC which is close to the ideal level for lead-acid batteries.

is a great site on how to handle batteries. They would like to sell you one of their chargers, but why not if you need something of that quality?

If the OP wants to use a better grade of charger then he will have to isolate each battery during charge, and then isolate the chargers during the load cycle. I don't see any way to do that with a single DPDT relay. Now a 4PDT relay could do it easily enough without any diodes, but the OP set the constraints, not me.

John ;-#)#

The problem as stated is impossible,

For charging you need to ground the negative ends of both batteries, connect the positive ends of both batteries together and connect the disconnected end of the charger to the batteries, that's three connections. and a DPDT can only make 2 connections at a time.

what do you really need?

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     ?

Buy a 24V charger.....or use 2 x 12V chargers in series so long as their outputs are isolated/floating.

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You won't get much into a lead acid by applying the resting offload terminal voltage.

it drops it by around 1v, down to about 0.65v as i approaches zero. More nonfunctional than nonideal, unless the charger is a relatively high impedance history piece, which is unlikely.

I think all modern ones do. Failure to do so is a liability issue.

sure they can. They just charge until not much current is flowing at 14.4v, then drop to float charge.

no, the flatter takes charge quicker.

too high for float charge

There are only 2 wires than need moving, the 2 connections of the upper battery. So DPDT should do the job.

NT

This statement is absolutely NOT true! It depends on the chemistry of the battery, even within the lead-acid types.

Sure it is.

No, only two are needed - the positive of one and the negative of the other. Battery 'A' positive terminal gets connected to +12V and Battery 'B' negative terminal gets hooked to ground. Battery A- gets switched from ground (parallel position) to battery B+ (series position). Battery B+ gets switched from +12V (parallel) to Battery A- (redundant connection).

Question not totally clear on load switching. You could connect load at node "NC" if you want load not to require manual connecting. ...Jim Thompson

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To those of us in my age bracket...

GREEN means inexperienced and/or incompetent.

nope.

You forgot to connect the charger. one of the requirements was that the charger only be connected during charging.

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Yes, that's the part I missed. I'd just unplug it. ;-)