Very basic dual battery question (1 Viewer)

[woytovich]

Trying to understand some basic DC electrical tech here.

Hypothetically, if I'm running with a single battery and it is fully charged and I then directly hook up a second battery (just pos<>pos, neg<>neg) that is fully discharged (to a level that has not damaged the battery) and then drive normally what will happen? What are the potential "bad things" that can happen?

Is water a good analogy here in that current will flow "downhill" to the lowest (charge) point?

Will the discharged battery take the output of the alternator plus some flow from the charged battery?

If so will the alternator supply the lion's share of the charge current?

I understand that an isolator would prevent the 1st battery from supplying current to the second in this case.

 

[george_tlc]

A charged lead acid battery sits at around 12.8V fully charged. Alternator, when it is running, will settle at around 14.4V (temp dependent and load dependent). So, with just the batteries connected in parallel (no alternator running), there isn't too much charging happening from the charged battery to the flat one since there's only 12.8V available - but of course there is some. This is why with a dual battery vehicle, you really don't want the potentially flat aux battery to be placed in parallel with the main battery until the engine has started and the alternator is providing reasonable charging voltage (hence products like ACRs), i.e. you don't want the flat battery to draw so much current from the main to prevent the vehicle from starting.

Once started, the alternator will be providing most/all of the charging current to the flat battery since it will be attempting to get to the 14.4V regulated output voltage which is well above the fully charged main battery's 12.8V open circuit voltage.

Lead acid batteries are constant voltage charged (14.4V typically) but that voltage can be initially lower (into a flat battery) since the charger (alternator or mains charger) will have some current limit that will cause charging voltage to be lower so as to not exceed the current limit. As the battery charges up, it will draw less current for an applied charging voltage, so the charger will raise voltage (up to the maximum 14.4V) while not exceeding the current limit. So, consider the charging as constant voltage/current limited. This for our 'dumb' alternator charging. A smart charger will go through various phases (which is why you read about thinks like, acceptance charge, bulk charge, float charge etc).

cheers,
george.

 

[woytovich]

Thanks George... that makes sense.

I am building a portable battery box that will serve to power "things" while camping and to run the ARB fridge when travelling so as to not risk the primary battery. I will probably get a DC-DC charger that will be the normal way to charge the battery when driving. Regarding the questions above I want to also to have a provision in place to hook up the portable battery (with full size battery cables sized depending on length) to assist in winching or starting if needed... and to charge if the DC-DC charger will take too long, say after a few days of depleting the battery with only a short time with the vehicle running to charge it back up.

Any thoughts on other methods to achieve these goals would be appreciated. (I have 3 vehicles that the portable battery pack will serve - I don't want to install a "real" dual battery system in each - I want to keep this portable.)

 

[george_tlc]

I'd put an anderson SB50 where you want to typically charge your portable battery and go direct to the vehicle +12V while engine is running and just use the alternator system to charge. Of course you could be looking at a few hours of driving to fully charge from flat.

For winch assistance, BIG cables and BIG connectors and short cable runs would be a prime consideration. This would be a tough one since big cables/connectors AND somewhere to mount the battery doesn't play well with the plan for the battery to be easily portable - which implies the battery lives in the back cargo area. I personally would just go with a big/good battery as the main for starting/winch duty and assume the portable battery is only for light accessory loads (the fridge etc).

I'd also look at getting a solar panel and charge controller to keep the battery topped up while camping. Something in the 100W range will easily keep up with an ARB fridge and LED lighting etc and keep your portable battery charged. Obviously solar assumes you have decent sun (versus wet winter camping)... You'll get bettery battery life if you can keep the battery mostly topped up.

A decent battery maintainer to extend the life of the battery when not in use would be a good investment.

cheers,
george.

 

[woytovich]

I was thinking the portable battery would be mounted on the floor behind the front seats (that's where the fridge is mounted) or next to the rear wheel well. (my rear seats are out permanently) The heavy cable run would be from there through the floor (bulkhead connectors?), along or inside of the frame and up to the pass front battery tray area. That would be about 10-12 feet... 1/0 or maybe 2/0 would be ok?

I COULD conceivably build a tray/mount for the portable battery to go right behind the driver side headlight (where most put a second battery in a 60) I'm using one of these:
Amazon.com : Newport Vessels Trolling Motor Smart Battery Box Power Center black : Sports & Outdoors

 

[JCruse]

Hey you crazy New Yorker!

I'm one too. (Long Island)


Several times I've hooked up a modestly discharged battery, think 12.7v, to a full one, 13.2v, through an ACR and it quickly pulls the one down till they're equal. You can watch a volt meter ticking down as you stand there. In maybe 10 minutes it's done.
Don't know if this helps but, just put it under FYI.

Where are you located?

 

[woytovich]

I'm in East Northport... you?
You should join the Gotham City Land Cruisers and get out in the woods with us this season.

 

[george_tlc]

Hey you crazy New Yorker!

I'm one too. (Long Island)


Several times I've hooked up a modestly discharged battery, think 12.7v, to a full one, 13.2v, through an ACR and it quickly pulls the one down till they're equal. You can watch a volt meter ticking down as you stand there. In maybe 10 minutes it's done.
Don't know if this helps but, just put it under FYI.

Where are you located?

12.7V is not modestly discharged. 13.2V is fresh off charging.

Nominal fully charged SLA is 12.8V (at 'room' temperature) after it has had several hours to 'settle'.

So, all you are seeing is surface charge (13.2V) being dropped down to a more nominal 12.8V range.

cheers,
george.

 

[JCruse]

Thank you George.