How about if I install a second, smaller, six-volt battery, wired in parallel with the original for charging, and with power source for AC and a cigarette lighter socket for cell-phone charging wired in series? Looks to me as though another battery would fit under the hood on top of the left fender, and certainly in front of the Florida room between the radiator and the grille.

Hey Bernard,

I only see the one post in this thread and don't know what your original thought was but based on what I read here, you're doing the same thing the military does when it runs 24 volts and 12 volts simultaneously in its M113 personnel carriers (this is the only one I'm really familiar with) They run four 6 volt batteries for twenty four volts to start the engine, but only two are used to supply twelve volts for the instrumentation, etc.

Ed

Ed

Ed,

Yes, that's the idea. I was hoping to hear from someone who has done a 12-volt conversion on a 47-48-49 car, and how they did it. I'd rather not put a second battery in the trunk; the trunk is suprisingly small and I need the space, and the wire runs would be long enough to reqruire some pretty heavy gauge cable. I know the science is good, but the mechanics of placing and locating the second battery, etc., are the question.

The Oldsmobiles and Buicks that came with Diesels also had dual batteries in them. They used them in parallel though and just had a huge amount of 12 volts. The reason I say this is because of the placement of their batteries; they were a mirror image of each other. Each sitting in front of the respective front wheel wells right and left. I drove a 50 Buick to high school for a while, but that's been so long ago, I can't remember where the battery was. Would it be possible to have one battery tray stacked above the second one? The trunk wouldn't be too bad if you have the room; lots of drag racers place their batteries in the trunk on the right side for extra weight - helping them with traction; a 0 or 1 guage cable could handle that. The newest generation of Rivieras had the battery under the back seat, like most boats do now. My Chrysler Sebring JXi convertible has a panel in the right front fender well you have to remove to access the battery. If you've got the ingenuity to think of it, you've got the ingenuity to figure it out. Ask some of the local street rodders what they're doing, they have space problems all the time. Good luck

Ed

Ed

Ample room; was hoping to hear from someone who had did it and liked or disliked the results the solution. (Battery on my car is low inside the right fender, close to the firewalll can't stack two there because the air cleaner is on the right side of the engine and intrudes into that space.)

Bernard
how are you intending to electrically seperate the batteries when you're going to have them wired series and parallel at the same time?

i'm looking at diodes on the second battery but that introduces low voltage problems. even if they do prevent a current backfeed (short) they are going to greatly reduce the charging efficiency of the secondary battery as each silicone diode ( http://members.aol.com/_ht_a/RAdelkopf/diode.htm ) will reduce your downstream voltage by .7 ... which on a 6v system is fairly significant. the 12v system will also be down to 11.3v but that may still be within tolerance of your 12v devices.

also, both diodes will have to be able to withstand the total amount of current that you want to use on your 12v system otherwise you're going to let the magic smoke out of them :grin: . i wouldn't be concerned about this 2nd point if you were just charging a cell phone but you're also talking about a voltage inverter and those range up to the hundreds of watts.

please test wire this circut outside the car and with light gauge wire before you attempt to implement it. if you do this with a heavy gauge wire in the car i could easily foresee cables getting arc welded and a battery exploding with unpleasant results for all concerned.

i'm thinking that if i were doing this myself i would prefer to install a 12v alternator in place of the generator, charge the two 6v batteries in series and then run the original car electrics off of the old battery. this has the added benefit of not really changing any of the existing wiring. isolate the positive output of the alternator to the top of the battery stack and tap the positive terminal of the original battery to connect it to the negative of the new battery and you're done.

bob,

It was my thought to "connect" the two batteries only through the wiring to the AC draws and the cigarette lighter receptacle, with separate ground straps. There would be two separate leads from the generator to the positive poles, each with its own voltage regulator, but no strapping of the batteries.

If isolation is needed, I'd look into a battery combiner rather than a battery separation (diode) system, for the voltage drop reasons you cite.

I think the concern here would be an increased charging demand, up to 2x normal, on the generator.

However, I do like your suggestion about going to a 12-volt alternator and charging in series. That certainly would obviate any problem about charging rate from the 6V generator, and would make the update from generator to alternator at the same time--thank you !

okay, first some background so you can tell how much (or little) i'm talking out of my @ss on this.

i do have a degree in electronics but:
a. it's a two year associates degree (means i'm a tech, not an engineer)
b. i graduated in 88
c. i've done practically no electronic repair or service since ... 93? 92? i taught myself autocad and did records for the phone companies (bell atlantic & GTE) for most of the intervening time.

now, on to your plans.

It was my thought to "connect" the two batteries only through the wiring to the AC draws and the cigarette lighter receptacle, with separate ground straps.

this is kind of what i was afraid of. the thing that you have to understand is that, electrically speaking, it doesn't matter how many different wires or cables you use or how far apart you put the junctions (barring runs of thousands of feet where the impedance in your conductors will be greater than the impedance in your loads) common electrical points are common throughout the circuit.

it doesn't matter how many seperate ground straps you use, ALL OF THOSE STRAPS WILL STILL BE COMMON ON THE FRAME/BODY/GROUND OF THE CAR.

battery A has it's neg tied to car ground (CG). so does battery B (otherwise it won't be able to charge from the generator). both A and B also have to have their pos terminals tied to the 6v out of the generator ... or, once again they won't charge. this is your parallel circuit.

now, in order to get to 12v you HAVE to connect the neg terminal of one of these batteries (let's say B) to the pos terminal of the other (series). otherwise you're not going to have anything but but two 6v sources with summing current (parallel) BECAUSE THEIR NEG TERMINALS ARE BOTH TIED TO CG. this means that the instant you complete that A pos to B neg connection (series), no matter where in the car you do this, battery A is being shorted through the CG and your ground strapsAND through the generator(yes, you'll burn that out if the car is running) between it's pos and neg terminals. that's why diodes would be required, to seperate the batteries AND the generator electrically. or, i should say, seperate them unidirectionally because you still want current to flow in one direction but not the other. the only good thing in this situation is that steel isn't a very good conductor and 6v is a low voltage so you might not actually burn your cables up or arc weld them as the short current will not be that great. let's say total resistance through the short circuit is 1 ohm. that gives a current draw of 6 amps ( voltage/resistance=current ) and power dissipation of 36 watts ( power=voltage*current, approx a 40w light bulb). not life threatening but not something you'd want to put your hand on either (due to heat, not electrical shock concerns, 6v just isn't going to feel like much because the resistance of your skin is so high). of course, the less the distance between the batteries, the lower the resistance (generally speaking) and the greater the current flow and power dissipation or heat.

I'd look into a battery combiner
and here i'm lost. maybe there have been advances in the intervening years that i'm not aware of but how do you 'combine' batteries without putting them in series? the more i'm looking at this circuit the more sure i am that either there's no way to make it work or i need to put a 3rd diode in there and i'm not seeing where it needs to be or in what orientation.

I think the concern here would be an increased charging demand, up to 2x normal, on the generator.
yes, this will certainly reduce the life expectancy of your generator if it doesn't cause it to fail outright. but it shouldn't be 2x because of the additional battery as long as both batteries have good charges when installed (and they should). most of the problem will arise with the added current drain from your voltage invertor. if both batteries are in good shape you'll probably see less additional draw than if you turn your dome light on.

speaking of which, what do you intend to run on AC? do you just not have a 12v charger for your cell phone or do you intend to install a TV as so many people are doing nowadays or what?

this also leads to another problem with multiple battery systems though. if connected in parallel there are often problems with the batteries feeding back on each other due to variances in the chemistry/reaction times between the batteries so even just parallel batteries are almost always seperated by diodes. check out a car stereo magazine or attend a car stereo show sometime and see how they do it. when you get to vehicles pushing stereos at thousands of watts they have to have parallel battery arrays simply because there is no feasible way to get the necessary instantaneous power draw off of even industrial strength alternators at 12v supply levels. for instance 1200watts/12volts=100amps ... and that's not even that large of a system. JVC has a six thousand watt sub-woofer ONLY amp. figure another ~2000w in full range amps & head unit.... :eek:
lessee, 8000w/12v=667amps. and we still haven't powered the headlights or anything else oe on the car. :eek: :eek: :eek: and those are just audio power outputs, not the current draw/loss within the amps. by the time you get done accounting for system inefficiencies you're well north of 700 amps draw. sheeeesh.

there aren't feedback problems with a series battery array because the batteries aren't on a common ground and aren't connected across the positive terminals and thus aren't really 'aware' of each other.

hope i've muddied the water enough. good luck with whatever solution you choose.

Okay, where's the blush icon? graemlins/smash.gif I was afraid you'd spot my erroneous thinking about not connecting the batteries in series . . . I realized it over breakfast this morning.

Battery combiners sense a rising voltage and connect two or more battery banks to charge from the same charging source; when voltage drops, they disconnect the banks so they don't discharge together. They're known particularly to owners of larger sailboats, who have to be very good stewards of electrical power aboard. They don't cause voltage drops as the diodes of a battery isolator do.

You're also right about variations in charging rates and so forth between two batteries. I'd overlooked that consideration, which will be in play because I won't have two identical batteries--not enough room under the hood for two of those huge farm equipment batteries.

When I spoke of making the transition from generators to alternators, I didn't have in mind having any AC current (as from an inverter)--just the newer technology of alternators.

All I plan to run on 12V is an air conditioner, a cigarette lighter socket for cell phone charging, and a stealth-installed basic am/fm radio. We're not into megawatts, though when I drowned out a bible-thumping street-corner student preacher this afternoon with Dee's Continental's radio up full blast, I would have enjoyed megawatts--the guy was still standing when we drove off . . . .

Thanks for taking the trouble for the extended answer. You're on point all the way along.

Battery combiners sense a rising voltage and connect two or more battery banks to charge from the same charging source; when voltage drops, they disconnect the banks so they don't discharge together.

hmmm. i'm still not clear on whether the 'combiner' is switching in additional batteries in parallel with the existing set and is simply disconnecting the backup arrays as a failsafe under low voltage conditions or whether you're actually stacking, say, an additional 12v battery in series when your v-source exceeds ~24.5 volts.

i should think that charging a 12v batt array at 21v (too low for two battery arrays in series) would be extremely hard on them.

don't be embarassed, it's better that these things be hashed out in public (the easy, cheap way) than experimented on in private (the hard, expensive, face saving way). :grin:

i'm sure there are/will be many people looking at this who will not have 1/10th of the electrical knowledge or experience of either one of us (and as for myself, i know i'm a bit out of practice). if we can save them a school-of-hard-knocks lesson i think it's been worthwhile, don't you?

Consideration of a battery combiner may have been part of my faulty logic to start.

Let us begin with a typical marine application where all components are 12V, and there are two battery banks (single batteries, or multiple 12V batteries in parallel, per bank). The combiner is connected between the charging source--be it a shore power battery charger, ship's alternator, wind generator, solar panels, or combinations thereof--and the battery banks. When it detects a higher voltage on the charge source side than in either (or any of) the battery banks, it makes a connection to allow charging current to flow in. When it senses a discharge or falling voltage from any of the banks, it disconnects the banks so that only one will discharge. With this sysem, use of lighting, pressure water, radar, or whatever in the "house" electrical system can't discharge the engine starting bank or battery; nor can prolonged cranking of the starting battery discharge the house bank so that VHF, running lights, and navigational equiment aren't put on short supply.

I suspect battery combiners use GFCI technology in reverse.

In application on a car with a complete 6V system, the combiner would first of all have to work at the 6V level, something I'm not sure of, and then would connect/disconnect two batteries in parallel when the generator was producing more voltage than either ( ? not sure if either or a leading battery) held. This does no good if the batteries are in parallel only, which, as you pointed out, wouldn't provide 12V power anyway.

But if the combiner could be installed between the two batteries as the series-forming strap, what then? It would close the connection when the neutral-pole battery was hotter than the hot-pole battery, providing 12V--again, it won't work, because the lead battery will not always be hotter than the following battery.

What if two combiners were installed between the batteries, forming parallel straps between neutral and hot poles, but reversed one from another, so that no matter which battery was hotter than the other, they would be connected in series? This won't work either. When the hotter battery is drawn down to the +/- margin built into or set into the combiners, they'd both open, and the series would be broken, and the 12V circuit would go dead, until 6V use of one battery or the other pulled it down below the margin set in the combiners. Besides, battery combiners are very expensive, and having two would be cost-prohibitive.

Your suggested solution remains the best: 12V alternator (or generator), two 6V batteries wired in series, 6V current taken of the leading battery only.

Bernard, there will be problems with tapping a load off part of a series battery string. The charging current supplied by the gen or alt will not be the same in each battery since some will be diverted into the 6V loads. As a result the batteries will not charge equally and will in time have different voltages on them. Guys have tried this by tapping 12V loads from 125V stationary battery strings or 125V loads from 250V UPS battery strings etc. It didn't work.

There are still a few 6V to 12V DC converters available on the Web. This device might be enough to run the cell phone charger or even a small AM-FM radio. Finding one big enough to run the compressor coil and blower of an a/c unit might be a problem.

Todd,

You are correct; but over time without drain (overnight, for instance), the two batteries should equalize, and even with different voltages due to the 6V drain from one battery during operation, the series voltage should still be the sum of the two voltages, less whatever voltage drop occurs across the strap (negligible), so I don't see a functional problem, especially if the gen or alt's charging wire is connected to the battery used for the 6V load, being charged "first" in the series. Besides, aren't there some three-pole 12V batteries out there form which 6V can be taken off by using the middle (hot)pole and the ground pole? I don't believe these batteries are suitable for automotive use, but if memory serves they exist, and seem to me to support the contention that a half-current draw can be successfully handled from a two-battery series. You say the examples you cited didn't work and I can't argue with that; but I still don't see why a 6V/12V division can't be made.

Bernard, yes I think a 3-terminal battery with a midpoint tap at 6V would work as long as it gets recharged from a 3-wire generator or power supply that more or less charges each 6V section independently. At one time there were stationary generators built with a 3- wire output that could do this task. It would be no trick at all to construct such a DC power supply to operate from utility power.

Now things get even more interesting...

If I understand your concept, the alternator would only produce a +12V charging current to charge the string of 6V batteries in series. Say the 6V draw from the car's loads totals 8 Amps for some period of time. Presume the alternator decides to produce enough voltage (maybe 12.6V) to charge the battery string at 15 Amps. The new 6V battery (with its + terminal tied to the alternator output) will then be charged at 15 Amps. The old original battery now in place will be charged at 7 Amps. The other 8 Amps will come through the new battery and on into the car's loads. Until something changes, the new battery will be charging at about twice the current going to the old battery.

As the new battery becomes fully charged, its voltage will rise to some value like maybe 7.5 V. The old battery will not be fully charged and would remain near 6.3V. The sum of these voltages will be the voltage at the alternator output as stated in your earlier message.

The alternator output voltage would rise a little to say 13.8 V in response. The alternator charging current will drop off to maybe 5 Amps now, which is fine with the new battery, it doesn't need the juice. Now the old battery will have to go into discharge to make up the difference due to decreased alternator output current. 5 Amps will come out of the new battery and on into the car's loads. The old battery then has to supply the other 3 Amps to make 8 Amps flow into the 6V loads. The new battery is then fully charged while the old battery is being drained. Yes, after an idle period, the voltage of the new battery would settle down some and drop. It will still be at rest in a fully charged state while the old battery is in a weakened state of charge.

Could you explain more about attaching the alternator or generator charging wire to the + terminal of the old battery? I can envision how that could signal the alt or gen to crank up the output current because the old battery needs it. But the new battery will outgas and overcharge and try to prevent an increase in charging current from passing through it.

Todd,

First of all, thank you for taking so much time with this.

Your example has an 8-amp draw from and a 15-amp charge to the new battery. It seems to me that the charging current would peel 8 amps off to satisfy the 6-volt load, and the excess current of 7 amps would flow into the two-battery bank and charge both with the new battery leading only slightly. Would not two batteries in series be essentially one battery with twice as many cells?

What am I missing now?

(I think we had cross-conception about old battery/new battery. In this reply I used your designation, and continue with yours here: Hot lead from alternator would connect to hot pole of new battery; ground of two-battery series would lead from negative pole of old battery; all car's 6-volt electrics would lead from the new battery and ground to negative pole of same battery; the 12-volt loads would lead from the hot pole of the new battery and ground to neg pole of old battery or to the frame.)