Which is the better battery configiration

[Campinfan]

I am trying to learn more about 6 volts batteries versus 12 volts. I know the 6 volts can be drained further and recharged more (better lifetime). At least that is what I think is true. But it seems 12 volts have more amp hours (or minutes) (I think). So if I got 4 batteries, would you get 4 12 volts and put them parallel or 4 6 volts and wire them in series to get 12 volts and then in parallel to double the amp time? And why? I may be venturing into the Motorhome world and I have many questions.

Thank you.

[pianotuna]

Blaster Man wrote:
I tried two 12 volts, but that was not enough juice to run the inverter...the third one fixed that.

How large was the inverter?

What type of battery bank?

[Blaster_Man]

I tried two 12 volts, but that was not enough juice to run the inverter...the third one fixed that.

[valhalla360]

Blaster Man wrote:
Call it what you like, my three 12 volt AGMs do fine on an all electric 45' coach if you do very little dry camping...we do virtually none.

If you never dry camp, buy a single flooded 12v as cheap as you can (around $100) and it will do fine.

There saved you an extra 120lb and probably $500-800 over AGMs which make zero sense in an RV unless you are doing barrel rolls on a regular basis with it.

Now back to your thread talking about actual apples to apples comparisons....

[Bruce_Brown]

^^^ I've been thinking about this - if you never do any dry camping wouldn't it make sense to buy the cheapest battery you could buy? You're not looking for quality, you're looking for something to take the voltage, and nothing else.

I do this at our hunting camp - I don't need a good battery, I just need something that will take a charge, it doesn't even need to hold it for more than a few minutes.

[Blaster_Man]

Call it what you like, my three 12 volt AGMs do fine on an all electric 45' coach if you do very little dry camping...we do virtually none.

[SJ-Chris]

Blaster Man wrote:
One often forgot about fact is that the 12v are very much lighter than the 6 volts. I went from 8 six volts to three 12 volts (all AGM) and my total weight went down a bit over 300 lbs.

12v AGM battery weighs ~63lbs (google)
6v GC battery weights ~60lbs (google)

Two 12v AGM batteries (~100AH + ~100AH = ~200AH) weigh ~126lbs
Two 6v GC batteries (~210-220AH) weigh ~120lbs

Eight 6v GC batteries provides ~840AHs capacity and weighs ~480lbs.
Three 12v AGM batteries provide ~300AHs capacity and weighs ~189lbs.
Yes, your weight dropped ~300lbs, but your battery bank capacity dropped ~540AHs. Your weight didn't go down because you switched from 6v batteries to 12v AGMs, it went down because you cut your battery bank capacity down ~540AHs (which is certainly the right thing to do if you don't need that much battery bank).

-Chris

[Campinfan]

It looks like the motorhome I may buy has 2 12volts in it now and they are 2 years old. I will keep an eye on them and when it comes time to replace them, I most likely will go for 6 volts.

[Blaster_Man]

One often forgot about fact is that the 12v are very much lighter than the 6 volts. I went from 8 six volts to three 12 volts (all AGM) and my total weight went down a bit over 300 lbs.

[valhalla360]

pianotuna wrote:
Properly set up is not possible unless you wish to buy a special purpose alternator. Or add a diode type device to the existing alternator.

The computer in the engine "looks" at the starter battery voltage. When the starter battery has recovered--charging is scaled back. Once that happens there is little or no charging of the house bank.

A dc to DC takes the incoming voltage and boosts it to an acceptable voltage for charging the house bank.

My dc to DC charger does a maximum of 20 amps. That works out to less than 5 amps per battery.

Because I have a larger than usual bank, I chose to set the output voltage of the dc to DC at 14.7 volts. Don't do this unless you have researched what the ideal charging voltage is for the house bank.

Except the house bank is connected to the starting bank when running, so the engine computer sees a lower voltage and keeps the amps up...not 100-200amps but if properly set up, it's still sending maybe 20 amps back which will largely negate the draw from the fridge.

If you want to fast charge, you need to upgrade the pathway to the house bank and make sure the alternator is one designed for continuous high output. Used to be quite popular with boats to upgrade to a 300amp alternator...now it would be simpler to add 400w of solar since most people drive during the day when the sun s out.

[pianotuna]

Properly set up is not possible unless you wish to buy a special purpose alternator. Or add a diode type device to the existing alternator.

The computer in the engine "looks" at the starter battery voltage. When the starter battery has recovered--charging is scaled back. Once that happens there is little or no charging of the house bank.

A dc to DC takes the incoming voltage and boosts it to an acceptable voltage for charging the house bank.

My dc to DC charger does a maximum of 20 amps. That works out to less than 5 amps per battery.

Because I have a larger than usual bank, I chose to set the output voltage of the dc to DC at 14.7 volts. Don't do this unless you have researched what the ideal charging voltage is for the house bank.

[valhalla360]

Campinfan wrote:
Thank you all, I am learning a lot. I ways thought my coach batteries recharged as I was towing. That is problem the root of my problem with my current rig. I was draining the batteries too low as I was running the fridge and driving long distances.

Properly set up, the alternator should be charging both starting and house batteries...no guarantee it's properly set up and often the charging line to the house battery bank is woefully inadequate. If you are drawing 20amps and only 6amps is getting from the alternator to the house bank, you will be running the house battery bank down...just question of if the bank can sustain the loss before you get into a campsite and hooked up to shore power.

Keep in mind, an old battery bank may only have a fraction of the original amp-hr rating.

[valhalla360]

Campinfan wrote:
The reason why I say four of either is because I would have room for 4. That is why I have the question of 4 12 volts which would give me 4 times the amps of one....or 4 6 volts which would effectively give me 2 12 volts. I hope that clears it up. If all things were even....I would have 4 x 12 volts @100 minutes a piece (just pretending here) so that would give me 400. If each 6 volt was 100, I would only have 200 because I know in series only the volts double not the amps but then they would be partially in series so then I would have the equivalent of 2 12 volt batteries @ 100 a piece for a total of 200. So it seems the 12 volt battery is a better idea until I think about how much lower you can drain the 6 volts.

OK, I see a lot of folks responding are messing up on units and it's causing confusion (ie: someone suggested starting batteries have 3-5times the amp-hours for 30 seconds...which makes no sense).

Voltage:
- By wiring them in series, you add the voltages (ie: 2 batteries at 6v puts out 12v power.
- by wiring them in parallel, the voltage stays the same (ie: 2 bateries a 6v put out 6v power.

Amperage: For house loads, it's all but irrelevant unless running a big inverter. Amperage (typically shown as CCA~Cold Cranking Amps) is usually of concern for starting an engine. No harm in using a starting battery to start a engine but for the same size battery bank, a deep cycle will have fewer CCA.

Amp-Hours: This is a measure of the amp draw times the duration in hours. If you pull 8amps for 10hours ~80 amp-hr. It's important to understand that to use it correctly, you have amp-hr @ X volts.

Watt-Hours: If you are comparing batteries of different voltages, it's often better to convert t watt-hr. Watts = volts *amps. So 12v @ 10amps = 120watts. If you draw that for 10hr, you will use 1,200w-hr.

Once catch is how fast you deplete the battery will impact how many amp-hr (or watt-hr) the battery will put out. This is why 5 minutes cranking a truck engine will kill a 70amp-hr battery. Theoretically, 500amps for 5 minutes will use just over 40amp-hr but the battery likely won't crank the engine anymore. (look up the phuerket effect if you want more details).

So let's take your example of 4 12v Group 27 batteries vs 4 6v golf cart batteries wired for 12v:
- The 12v batteries will have around 90amp-hr each. Since you aren't changing voltage, you can multiply to get the total amp-hr ~ 360amp-hr total.
- The 6v batteries will have around 220amp-hr each but when you merge 2 to get 12v power, the amp-hr stays constant. Since you are combining 2 sets of 2, you wind up with 440amp-hr total (importantly @12v)

440amp-hr is significantly more tan 360amp-hr and hence why the 6v batteries are preferable.

Another way to calculate it is watt-hr:
- 12v @ 90amp-hr * 4 batteries = 4,320w-hr
- 6v @ 220amp-hr * 4 batteries - 5,280w-hr

You also get a little boost from the phuerket effect mentioned above as you are drawing at a slower rate (relative to the battery bank size) with the 6v.

Someone suggested it's OK to take the bank down to 20%...if you immediately start charging, it's only minor damage but if they will sit at 20% for hours, yes it does degrade the batteries.

PS: Lithium is very deferent in that they are largely unaffected by taking them down as low as 10%. This allows you to get away with a smaller battery bank in terms of amp-hr for the same usage. It does come with some catches in terms of management and proper charging or you can ruin an expensive set of batteries easily...but it's getting better.

[Campinfan]

Thank you all, I am learning a lot. I ways thought my coach batteries recharged as I was towing. That is problem the root of my problem with my current rig. I was draining the batteries too low as I was running the fridge and driving long distances.

[SJ-Chris]

Campinfan wrote:
So Pianotuna, are you basically telling me that I could get a DC to DC charger and then that would take the starting (chassis) batteries and use them to charge the house batteries? And of course the alternator would recharge the starting batteries.

The chassis batteries do not charge the house batteries. It is the chassis's alternator that charges everything (chassis battery, and house batteries if you have a DC to DC charger).

-Chris