Why is taking 12v from one batt bad?

[ratat98]

Ok, I've done a lot of searching on the site and I know it's not a good thing to take the voltage from one of the two batteries to use for 12 volt accessories, and I need to get a converter, equalizer, etc... But I have yet to find the reasoning behind why it's bad. I'm not an ace at searching this place yet so I'm sure I've just missed it, so if someone could please explain to me what will happen if I use one battery for accessories I would greatly appreciate it!!
Thanks in advance all!
-Rat

 

[uscgmatt]

The 24v alt charges both batts evenly. When you pull a load off one batt the alt attempts to charge them both resulting in the 12v one being discharged and the other one to get overcharged. Hopefully this helps.

 

[cranetruck]

The 24v alt charges both batts evenly. When you pull a load off one batt the alt attempts to charge them both resulting in the 12v one being discharged and the other one to get overcharged. Hopefully this helps.

Or putting it another way, if one battery is allowed to be "drawn down" more than the other, its internal resistance will be greater. When charged in series, the battery with the higher internal resistance will have a higher voltage across it (current is the same for both) and may become overcharged while the other battery won't be charged at all.

Batteries that are charged in series should be matched in every way possible.

 

[Rustygears]

The 24v alt charges both batts evenly. When you pull a load off one batt the alt attempts to charge them both resulting in the 12v one being discharged and the other one to get overcharged. Hopefully this helps.

The alternator does not charg both batteries evenly. This is the reason that one bad battery prevents the pair from charging. It is also why some folks recommend peridically disconnecting and charging each separately

The battery with higher internal resistance determines the performance for the set. When you draw off of just one battery, you increase the effective internal resistance of that battery (ohms law) and impair the function of the 24v system. This is negliable for very small loads, but is a really bad idea for heavy loads

 

[uscgmatt]

I guess I should'nt have used the word evenly. I meant both batteries get the same charge rate.

 

[Truckoholic]

If you just want an accessory port to charge your cell phone, or run a GPS, or even a stereo as long as you aren't running a big amp and subwoofers, there would be no problem at all.

 

[Stalwart]

If you just want an accessory port to charge your cell phone, or run a GPS, or even a stereo as long as you aren't running a big amp and subwoofers, there would be no problem at all.

Sorry but that is just plain incorrect. It WILL be a problem, it will just not occur as soon.

 

[Jake0147]

When the generator charges, it does not "see" two batteries, it sees only the voltage at the alternator. It's job is to keep that constant.

When two identical batteries recieve an identical draw and and identical charge current, in theory they will remain identically charged.

When you "steal" current from one battery, it recieves less charge.

The alternator does not see "one battery less charged", it sees the sum total of the two to be less, and will increase it's output until the demand is met.

The (crude) diagram shows a two battery charging system, and a two battery charging system with a twelve volts stolen from one battery. This is initial. You can see that the current to the number one battery is cut in half. (That's a pretty dramatic example). With sustained use (time dependant on the amount of current "stolen"), it's reduced charging rate will cause it to become less charged than the other, which will become overcharged.
With the batteries beginning at 12 volts each, as time goes on they will become 11 and 13, 10 and 14, 9 and 15, 8 and 16, and so forth and so on... Always a sum total of 24 volts, but no longer equal. The battery with stolen current is abused by keeping it poorly charged, and the battery without stolen current gets really abused by overcharging it.

 

[ratat98]

Wow thank you guys! It does make sense now! I'm thinking about putting a separate alternator and battery in to run a 12v system. I don't want to run anything big but a few lights (which I guess could be considered big), and charge my cell, etc. I definitely won't take power off one battery now though!

 

[wbdisco]

I am thinking of possibly running CB and 12v fuse block off of one battery with a disconnect. I know I am new to the SS scene, but some of the guys who I know that have been collecting military vehicles for 40 years now never seem to have that big of a problem when running radios, CB, etc.. I mean besides there aren't any electronics running on a diesel except the fuel pump and lights right??

 

[doghead]

Your starter is pretty important.

 

[Stalwart]

I am thinking of possibly running CB and 12v fuse block off of one battery with a disconnect. I know I am new to the SS scene, but some of the guys who I know that have been collecting military vehicles for 40 years now never seem to have that big of a problem when running radios, CB, etc.. I mean besides there aren't any electronics running on a diesel except the fuel pump and lights right??

Do what you want, you have to pay to replace the batteries before their time. You CAN do it but it WILL shorten the life of your batteries, PERIOD! If the draw is really, really low, it might make little more than a few months or year difference. Bigger loads will kill your battery VERY quickly.

 

[unispeed]

#They do make nice 24vdc to 12vdc converters before i went with a alternator
but with out getting shot my M35a3 came off the base with radio plugs and 12v outlets pre installed....off of 1 of the 12v battery

30 amps for 30 bucks
stereo grade!
Pyle 24V DC to 12V DC Power Step Down 720 Watt Converter w/ PMW#

 

[Varyag]

If you switched the batts every once in a while, would it help?

Would a 12 charger on that batt every once in a while also mitigate this?

i am all for getting a step down. I just need to get it in.

 

[doghead]

This is kinda like saying, if you eat alot of Taco Bell you will get diarrhea. If you take a bunch of Tums it will help. But if you just don't eat at Taco Bell (you have other better choices that may cost more), it's a non issue.

You could just charge each battery separately after each use. But who wants to do that...

 

[unispeed]

yes the load and duration on is part of the equation or made it a point to run the truck with the 12 volt equip off for a period every other trip would be better for charge equaliztion.... (But i am not any eletrical gooooorooo) you have to watch and use the battery whose ground (-) is connected to the truck frame and use the plus(+) from that battery for your 12v other wise if your equip gets on the chasis it could see 24v......but being that your stereo will be used when the truck is off alot it should not be a big bad load the way I way i work is if it eats a battery then i buy a new one and go to plan b How ever it is always sweeter to be prepared for the possible plan "B" so it is no a total surprise

 

[212sparky]

#They do make nice 24vdc to 12vdc converters before i went with a alternator
but with out getting shot my M35a3 came off the base with radio plugs and 12v outlets pre installed....off of 1 of the 12v battery

30 amps for 30 bucks
stereo grade!
Pyle 24V DC to 12V DC Power Step Down 720 Watt Converter w/ PMW#

I used the same converter in my truck to charge a seperate 12 volt battery that i run my 12 volt loads off of. I also ran my 12 volt back up lights in series so i could run them off of the 24 volt system to keep the load off of my 12 volt system.

 

[fstfrdy]

sorry for the crude drawing but this is how my accesory plu is kooked up. was like that when i got the truck and has worked fine for thetwo plus years ive had it

 

[Truckoholic]

Sorry but that is just plain incorrect. It WILL be a problem, it will just not occur as soon.

Okay sorry. Let me rephrase that............. The problems will be so negligible that to a lot of people like myself, it is not worth the fuss of doing it the "Right" way..

I've been doing it in all of my big trucks for years, and I only have to replace my batteries once every 6 months or so.

 

[Stalwart]

I used the same converter in my truck to charge a seperate 12 volt battery that i run my 12 volt loads off of. I also ran my 12 volt back up lights in series so i could run them off of the 24 volt system to keep the load off of my 12 volt system.

For small to moderate loads your approach seems best, especially if you consider using a small dry cell motorcycle battery as your 12V storage. Mount 'em in any position and can't leak, 'cus they are DRY!

Next in my opinion would be a battery equalizer.

Followed by a second alternator . . . and finally a 12/24 dual voltage alternator like used in some HMMWV's. The last is the most expensive but the coolest.