Batteries

There was some talk earlier this year about using two parallel 12v. batteries (instead of two 6v. or a single 12v) in a 140 or a 150. The discussion mentioned better cranking as a benefit and the easier availability of 12v. rather than 6v. I thought I had saved those comments, but can't find them. Has anyone tried it? How did it work out? - Sam Bell, 140 FHC

Two 12's in parallel will definitely give you more capacity and cranking ability but why do you need that. If the car doesn't start a a couple of spins, you've got a problem that should be solved before driving off. The 6 v's are in series and to change over would necessitate some rewiring. The problem with two 6's is that one will age differently than the other and unless they are replaced as a pair, you will have "charging" problems and constantly be replacing difficult to find batteries. Go with one 12 and put it on the passenger side. - George Badger

Two parallel 12V. batteries does not produce 6V. It just provides twice as many amp hours for a 12V system. It's the same as using jumper cables to start a car. Putting 12V on a 6V starter will probably make it crank faster for a short time - until it starts to smoke. I recommend against trying it. - Bruce Cunningham

Whoops - to bad you can't take back a note 5 seconds after sending it - I just realized we are not talking about a 6V system at all. I guess I got my wires crossed with my 29 Chevy. Two 12's in parallel will be an advantage if the amp hours are more than the pair of 6's - which may or may not be the case as amp hours depends pretty much on size of the box. - Bruce Cunningham

sorry - now I want to take back both notes. What relates to "cranking" is power which is volts multiplied by current. The amount of power in a battery is related to the size of the box. It is possible that two twelves in parallel could have the same power as two sixes in series because the cells (2V each) are larger in the sixes. Batteries are rated in amp-hours which relates to power for two batteries with the same voltage. To compare the series/parallel alternatives, add the amp-hours for the parallel twelves and compare it to the individual amp-hours in the series sixes. If the total of the two twelves is more than the individual sixes (not added), then there will be a power advantage. In any case, I agree with George - you shouldn't need more cranking power. If I have to crank mine more than about three turns when it is cold, I fix it. When it has been running within the last 3 or 4 hours, I don't even wait for it to start to release the button - I know it will be running if I just press the starter and release it immediately. - Bruce Cunningham

When I put the XK150 that I bought back together, I had a terrible time getting the engine to crank reliably with a single 12v battery (New Sears Die-Hard). I could charge the battery up and it would crank OK for a day or so, but then would need charging again. I put a second 12v in parallel, and it starts wonderfully. I do get some funny readings off the ammeter, and haven't figured out yet if it's the meter or some problem related to the batteries (seems unlikely). But it sure solves the cranking problem. - Good Luck, Steve Crounse

Two six volt batteries in series will have slightly MORE capacity than two twelve volt bateries in parallel. The six volt batteries contain 3 cells and two cell separators while the twelve volt batteries contain 6 cells and 5 separators. If the two batteries occupy the same volume there is more lead and acid in a 6 volt battery, giving it a higher capacity. That should not be the deciding factor, however. None of my jags have ever required more than a single revolution to start the engine - a quick punch on the button is all it takes. Price and availability are more compelling reasons to choose a battery size. In my case I have two twelve volt gel-cells in my XK120 because I got them for free. Like I said, price and availability. - Mike Eck

I obviously didn't express myself very well. Lead-acid batteries consist of multiple cells, and each cell produces a nominal 2 Volts. Even though a battery looks like a single box, inside that box are cells that are electrically and chemically separate from each other. If you were to cut the top off a battery (like I just did in preparation for writing this) you would see that a 6 Volt battery has three cells and two plastic separators to keep the cells apart, and a 12 Volt battery has 6 cells and 5 separators. The separators are made of plastic and are about 1/4 inch thick. They are necessary to insulate the cells, but otherwise contribute nothing to the electrical capacity of the battery.

If we assume an XK battery has internal dimensions of 7 X 7 X 7 inches, the total volume of the battery is 343 in^3. A separator would have dimensions of 7 x 7 x .25 , or 12.25 in^3. In a 6 Volt battery the two separators would occupy 24.5 in^3, or 7.1% of the internal volume. In a 12 Volt battery the 5 separators displace 61.25 in^3, or 17.8% of the total volume. The 6 Volt battery has 92.9% of its volume left for plates, while the 12 Volt battery only has 82.2% left, or 11.5% LESS capacity than the 6 Volt battery. (92.9-82.2)/92.9=.115 An example from real life... According to the standard for automotive batteries DIN 43 594,

Capacity Designation V Ah Volume M^3 Weight kg

6 V GiS 180 6 180 .01275 30 12 V GiS 180 12 180 .03647 64

Two of these 6 Volt batteries would weigh 60 kg. and have a volume of .02550 cubic meters. As you can see, to achieve the same Ampere-hour rating a twelve Volt battery takes up .01097 more cubic meters and weighs 4 kilograms more than two six Volt batteries of the same type. Table from _Battery Technology Handbook_, H.A. Kiehne, VARTA Batterie AG, Marcel Dekker, Inc, N.Y. 1989. It's amazing how many separate disciplines are covered under the title of Electrical Engineering. A few years ago I designed a computerized speed control/charge gauge/recharger for an electric golf cart manufacturer, but to me, RF is black magic. - Mike Eck, '51 XK120 OTS, '62 3.8 MKII MOD

added 1/10/99...

Ever since I had my XK engine rebuilt with 30 thou over pistons my two six volt batteries have not provided enough crank for cold morning starts. Any suggestions,other than mounting a big battery in the boot? Is anyone familiar with the Optima six volt dry cell,which can be stuck in the box on it's end and allegedly provides over 800 cold crank amps? (I'd have to use two in series) I've considered trying two new conventional six es-when you put them in series you double the voltage but do you also double the amps? Thanks for ANY help. Please respond to the below e-mail address, not the sender address. - Jay Fox

Jay, If your two six volt batteries are good and the cranking is weak have you checked all the heavy duty wiring between batteries and the solenoid and then on to the starter. Are you sure the starter is OK? Is the connection from the battery to ground good and also was the engine ground lead replaced? Two six volt batteries in series doubles the voltage but the current remains the same and if all else is OK the current will be determined by whichever battery has the highest internal resistance. - Eric Capron

I am the new owner of an XK120 FHC. All of my other cars have a 12 volt battery that I charge easily when needed. With the two 6 volt batteries in series, must I charge each separately after disconnecting them or can I charge both together leaving them in series with the 12 volt charger. It would seem to me that the latter is appropriate since the two in series is essentially the same as a 12 volt battery in one casing. Interesting. - Melvyn Goldberg, XK140 OTS, XK120 FHC, Austin Healey BN4

If you look at the top of some batteries, you can see things that look like jumpers thta go from cell to cell. Each cell has 2 volts. When you connect two sixes together, you are doing the same thing as putting all six cells in one box with the cable from battery to battery doing the same thing as the jumber from the 3rd to the 4th cell in the 12v. battery. So the answer is charge the two sixes together just as you would charge a twelve bu connecting the charger to the outside poles of the pair. - Bruce Cunningham

Good afternoon, Either will work. I agree the later is best if for no other reason than it's easier. No disconnect/reconnect to deal with. - Ken Boetzer

I usually charge my (2 x 6v) batteries in the car, connecting one charger lead to one side of the starter solenoid and the other side to earth e.g. the engine. - Regards, John Elmgreen

Yes, it is okay to pretend that it is a 12 volt unit and go across the positive end and the negative (follow the usual spark precautions). BUT one thing to be aware of; if the batteries are not of approximately equal discharge and similar state of plate condition, they may suffer unequal charge and dry out the plates on the 'stronger' because it will reach its charge sooner. - Barry Goldman

You can charge both batterys as "one" 12 volt, but first check each battery separately or you will wind up cooking the good battery. It is quite common to have on good one and one bad. Also make sure your connections are all clean and tight. really check the contact point for the battery to chassis ground. I like to use a conductive never seize {you can usally find them at marine dealers} at all my battery connections. No more corrosion. - Ralph @ cloverleaf auto

Hi Melvyn Goldberg & all -- My original Mk IX, RHETT (792817 BW) also has twin 6vs -- to charge them, simply insure that the batteries are connected in series for 12v output, hook up to - going to starter solenoid and + going to ground. Set the charger at 12v; it'll charge BOTH 6vs just fine! I hope this helps -- Larry Martz [USA, Calif.]

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