Limited electrical knowledge

[Urban Hermit]

Originally Posted by twinboat

In starting applications a 6 volt motor will draw twice the amps as a 12 volt starter motor. As engine compression increased more powerful starters were needed. 12 volt systems were developed for that along with high cranking amp batteries.

Heavy equipment use 24 volt systems to start large engines to cut down on the amp draw. They don't use 24 volt batteries but 2 12 volt ones in series. That also keeps the starter cable size smaller.

The Cat V12, C27 1000 HP engine uses 2 24 volt starters.

Given the same load, the higher the voltage the lower the amps and the lighter the needed wire. Chris Craft went nuts with this in the 50s and 60s; not sure about after that.

[SLOweather]

One thing I didn't see mentioned. 2 12 volt batteries in parallel, unless closely matched, ca self discharge each other when disconnected from load and charger, as their terminal voltages never exactly match.


I had this happen in our Suncruiser, and installed a Perko Switch. It had 4 positions, and allowed me to use 1 or the other battery at a time, both together, or both off.

[Urban Hermit]

Keep in mind that a wet cell (at least) 12V battery is flat dead at 10 volts.

[theoldwizard]

Originally Posted by Urban Hermit

Oops -- meant to quote Gary RVRoamer.

Excellent answer. Quick note: 12V deep-cycle batteries are always available from marine suppliers, but not at a cost savings -- usually the opposite. (Boats, planes, race horses . . . )

Just remember that "dual purpose" batteries are NOT true "deep discharge". The only true deep discharge batteries are golf cart batteries, either GC2 (6V) or GC12 (12V), OR solar batteries.

Batteries for electric fork lifts or floor scrubbers are great, but they are much too large and heavy.

[Urban Hermit]

Originally Posted by twinboat

In starting applications a 6 volt motor will draw twice the amps as a 12 volt starter motor. As engine compression increased more powerful starters were needed. 12 volt systems were developed for that along with high cranking amp batteries.

Heavy equipment use 24 volt systems to start large engines to cut down on the amp draw. They don't use 24 volt batteries but 2 12 volt ones in series. That also keeps the starter cable size smaller.

The Cat V12, C27 1000 HP engine uses 2 24 volt starters.

Quite so, but are any of us considering 6-volt systems, or only the efficacy of putting two 6-volt batteries in sequence to generate 12 volts?

[twinboat]

Originally Posted by Urban Hermit

Quite so, but are any of us considering 6-volt systems, or only the efficacy of putting two 6-volt batteries in sequence to generate 12 volts?

My post pertains to post # 12s question. Read ALL of the posts before comment.

[Urban Hermit]

Originally Posted by twinboat

My post pertains to post # 12s question. Read ALL of the posts before comment.

My deepest and most abject apologies. I failed to indicate that my comments wasn't meant for you alone.

[djbckr]

Originally Posted by LJowdy

Probably because the 6 volt batteries have more amp hours per battery.

As an example, (2) 6 volt batteries with 100 amp hours wired in series produce 12 volts and 100 amp hours.

(2) 12 volt batteries with 65 amp hours each, wired parallel will produce 12 volts and 65 amp hours.

No, this is not correct. Two 12v batteries are twice the power of one 12v battery. Two 100Ah 12v wired parallel is 200Ah 12v.

By the way, it is not a problem to have two 12v batteries parallel vs two 6v series. Add up the numbers appropriately. Often the 6v batteries are deep cycle whereas the 12v are not. Just find 12v deep cycle and you are good.

I have two 12v parallel in my rig (total 400Ah) and they work great.

[Piros1]

Ditto!

[MBrant]

12 v application use MUCH smaller wires than 6 v for the same power load. Wire was expensive and in short supply at that time.

Also, when a motor starts up, it uses 7-10 times the power it needs when it comes up to speed. That's the whole idea behind "soft-start". However, starter motors need maximum torque as soon as possible, so they have big honkin' wires.

[wmalexaner]

All the discussion above is good (with the 12 Volt Amp-hours correction).

Regarding the car battery question, it is true that 6 Volt starter wiring requires heavier wire for the same power as a 12 Volt starter. Also, the 12 Volts can be delivered to the starter more efficiently, for the same reason that long-haul electrical lines are higher voltage than we use in our homes. It's called I2R (I squared R) loss. Aside- it's also why automakers keep talking about moving future cars to 48 Volts. Actually, my old MGB DID have two 6 Volt batteries as you suggest, back when they were converting to 12 Volts in England. They were heavy and had to be located in the rear.

Another side benefit of the 6 Volt series arrangement is that if forces exactly the same current to flow through both batteries. At these low voltages and high currents it is very difficult to get two parallel batteries to share the workload, and it is often true that one battery works harder than the other when they are paralleled. (There are excellent articles on EXACTLY how to route the wiring to minimize this problem, but it can not entirely be eliminated.) The series connection tends to keep the load a little more balanced, although it is not perfect either.

That's what you get for waking up a retired Electrical Engineer!!!

[jerichorick]

Originally Posted by seb1899

Thanks to all who have 'helped to educate me" If two 6's are the best way to go because of plate size capacity etc. why did the automotive industry abandon the 6V battery?

My old old ('53 Plymouth Mayflower) is 6V and the hand held spot light that is a genuine Mopar accessory is 6V.

I am not trying to promote an argument just wondering why 6V was abandoned if it is better in this (RV Application)

It is a matter of power. For the same power consumption, the small the voltage the more current required. That equals bigger wires, more battery power, heaver everything to handle the larger current, etc. Going to 12V cut these things in half by theory. The industry could have gone to 24, 36 or 48V if they had chosen to do so. Some how they felt that 12V was the safest/best choice. Go figure.
By changing your 6V components, starter, generator to an alternator, bulbs, etc you could upgrade to a 12V system. Lots of work.
Hope this makes sense.
Happy trails,
Rick Y

[THenne1713]

Originally Posted by seb1899

First let me start with the fact that I know nothing about electricity except ON and Off.

The house batteries in my unit are 2 6V in series and I was wondering why it is 6V in series rather than 2 12V in parallel?

Is there a good reason for this?

Because of (heavier plates, I believe?), two six-volt in series has higher AMP-HOUR RATING (20-30%? without looking) AT 12V than One 12vdc RV/Marine dual purpose.... at a slightly higher cost?. The other LIMITATION is physical space, four x6v maybe better than 2x12v, but no room for four, or no room for two 12v, but YOUR NEEDS and YOUR $$ really dictate. If you do NOT boondock much, 1x12vdc maybe fine for limited use; TWOx6v can cost 2x as much and 4x6v can cost 4x as much. HOPE this helps... you can ADD batteries and trays and solar and other accessories and spend $1,000-$4,000 doing so..

[picker]

WRONG !!

In parallel.........the current adds up.

In series.......the voltage adds up.