Wiring configuration for 2nd battery

[sergei]

Thanks for such an insightful response!

If a lead acid deep cycle bayterr is 105amp hour rated. Does that mean at 50amps it would take 2 hours to charge?

Nope, there is charging efficiency as well, what would happen is that some current will be lost as heat.
What they mean is usually 5A load will last 21 hours (10A load will last less than half of time and so on, as higher the current less efficient the battery)

Be careful with amp hour ratings,

sure it may do 5A for 21 hours, but at the end of it it will be flat as in close to 0 volts, about 5hours into the 21hours it will be too flat to run a TV, start a car, do anything that requires kore than a few volts.

I totally I agree on that, hence the wording .
Most car batteries are rated Ah at 5A discharge (from memory).
Lead Acid is in particular bad as it will drop voltage almost linearly with loss of charge, and technically you cannot use it fully.

[matt dunn]

my mate's camper van has a second battery on a manual switch,

Manual sw, is ok as long as you remember to manually switch it.

We never rely on manual switching as the oldies that own campervans can never figure out them complicated switch things,


A VSR is the best way, but diodes are ok too but need to do something with the sensing wire too.

[cat007]

Why would you purposely drain a battery dead I mean I know you want to test your system out, but it can't be good for the battery to try and kill it.

Couldn't you have just disconnected the battery or something?

Because I tested it with a meter and checked everything - and it all 'appeared' to be sweet but I just wanted to do a real life test to make sure everything did actually work ok.

[cat007]

A blocking doide isn't the cleverest of ideas because if it is in fact a diode there will always be .7 volt drop across it. That means that when the alternator voltage is say 14.2 max, the highest that the battery can charge to is 13.5 so will never fully charge (in reality it's probably a FET controlled by a diode but you won't know without tests or spec sheet).

The best way is either a manual switch, or if I was doing it, I'd build a voltage controlled relay to switch it in automatically when it's on charge, and to isolate it when off charge, with a manual reconnect for starting.

As mentioned before, my mate's camper van has a second battery on a manual switch, it's never let him down, nice, simple, cheap and effective.

There's also a minimum cell voltage that you should NEVER drop below.

Steve

That's why the sense wire for the alt is after the diode, on the battery side, that way the batteries are getting (as long as both sides of the diode have the same voltage drop which I haven't checked yet actually) ~14.2 or whatever full charge voltage is. Just means the alternator has to work just that little bit harder.

[cat007]

my mate's camper van has a second battery on a manual switch,

Manual sw, is ok as long as you remember to manually switch it.

We never rely on manual switching as the oldies that own campervans can never figure out them complicated switch things,


A VSR is the best way, but diodes are ok too but need to do something with the sensing wire too.

EXACTLY! Most times I'm going to be drunk or other when we're stopped - and will probably start the van randomly to charge the batteries which means I'll have to manually switch it back, then, I'll have to remember to switch them back. I just don't trust myself, or the misses, to remember to do this