Battery physics

[ciderman]

Ok , I want to run 3 Optima yellow tops all linked parallel , The two new ones are 765cca and 55ah , would it make any difference to link the both of them to a third(exsisting) yellow top which is rated 690cca and 55ah .

I was concerned about them having different cca ratings .

Jase

[Kim Horsevad]

Ok , I want to run 3 Optima yellow tops all linked parallel , The two new ones are 765cca and 55ah , would it make any difference to link the both of them to a third(exsisting) yellow top which is rated 690cca and 55ah .

I was concerned about them having different cca ratings .

Jase

Short answer: You can connect them without problems - at least for the short term, but the batteries will try to "destroy" each other over time. Best practice is to fit some kind of split charge system, such that the batteries only are connected when the alternator is charging.

(Simple versionof a split-charge-system: trigger a 40amp relay (or winch solenoid) from D+ on alternator).

If you want a more in-depth answer let me know.

[treebloke]

Why not fit second alternator for the third battery or have you already thought of that.

We put one on Berts 300 tdi which is so simple to do and gives you the extra back if the others go pear/cider shaped.

[Bull Bar Cowboy]

Jase,

For your application it will be fine .......................

I guess you need to know that the 'stronger' batteries will always discharge into the weakest in that configuration, but if they are all OK (ish) now, then the balancing act between the batteries will be so small that you wont have anything to worry about........

I can understand why you may not want to go down the road of split charging one of the batteries (that would always give you a good engine starting battery) as it will add unwanted complication to your challenge truck ........ i.e. just something else to go wrong when you least need it to.

So yes, parallel all three together with decent sized straps (bigger the better) .................

Ian

[ciderman]

Thanks Ian

[Orgasmic Farmer]

Thanks Ian

Just to ressurect this topic....

I have just fitted a X-charge to the road truck and have been thinking about fitting something similar in the challenge truck after one of the batteries destroyed itself after a period of several months of not being used. This got me thinking that due to the current set up with the batteries in parallel linked with 70mmm cable then even a beefy relay like Si sells would not be up to the job. So would something like an Albright be a suitable relay for such a high power application (just wired in one direction obviously). Or perhaps one of the many spare warn solenoids I have lying around could be used? If I understand the principle of the split charge all you are doing is introducing a switch between the 2 batteries that is only turned on when the alternator is charging, ie. the engine is running. An albright is only a HD 2-way relay after all is it not? What I want is for both batteries to work together at full power when the engine is on but to be isolated when the truck is left idle for weeks on end.

I don't really have a great grasp of electrickery so please tell me if my thinking is flawed...

[JST]

s on end.

I don't really have a great grasp of electrickery so please tell me if my thinking is flawed...

no your thinking is flawed you really dont have a great grasp of electrickery

whether what you suggest may work i have no idea but understand what you are suggesting and am interested to know

[Guest WALFY]

I have a heavy duty starter solonoid that links the 2 batts together on the push of a button. I had burnt out a couple before going to an auto electrical place in Westbury. Told them what I wanted to achieve and they gave me the heavy duty model.

[ozzie1989]

Look at marine places, they sell 300amp to 500amp voltage sensing split charge relays for around £200 if you want to pay the money...

A solenoid will work however you'll get a voltage drop which will mean that the batteries never fully charge...

[robhybrid]

Whilst playing with warn type solenoids I have found.

The solenoid is intended for intermitent duty.

The solenoid coil burns it's self out if left switched on for long periods of time, the longest I managed was 25min and the solenoid casing was very hot.

[FridgeFreezer]

An X-charge is just a big HD relay, fitting a bigger (and vastly more expensive) one is probably not going to make much difference as it will only see full current if you manage to have one battery flat enough to draw 180+ amps (the working limit of the X-charge) out of the good battery, without lighting the alt charge light, when the X-charge switches on. I would think this a rather unlikely set of circumstances.

[TSD]

Whilst playing with warn type solenoids I have found.

The solenoid is intended for intermitent duty.

The solenoid coil burns it's self out if left switched on for long periods of time, the longest I managed was 25min and the solenoid casing was very hot.

I have some Trombetta solenoids in Warn packaging which are rated for continuous duty at 125A, intermittent to 250A. They do get hot though.

I use one for linking two Optimas together, and it's worked fine for a few years. Hold in voltage is only 2.5V, so ideal for jump starting and the odd bit of winching without the relay dropping out.

[simonr]

Although most of it has been covered already, the issue with connecting in parallel is that when you are not using the vehicle, small currents continue to flow between the batteries because they are rarely perfectly matched. Although these currents are small and will not in themselves make much difference to the battery charge state, they cause deposits to form on the plates over time. This causes a cumulative decrease in capacity.

This is the main reason you need to replace the batteries in computer UPS's and emergency lighting systems every couple of years, even if they have never been used. The continuous trickle charging - small charge current - causes the same degradation.

If you can get two or more well matched batteries, ideally from the same batch, the damage may never happen or take many years - but I have seen it happen in the order of months with badly matched batteries - often killing both of them!

If it's capacity you need, I would suggest just buying a bigger single battery - which will likely cost less and certainly weigh less.

As mentioned above, winch type solenoids are not good for split charges as the coil is not continuously rated and burns out fairly quickly. Allbright make some nice single contactors which do have continuously rated coils - so, if 180A is not enough, they are what I would recommend.

In practice, although the peak current can be quite high, the duration is very short. I tried measuring the current flowing between one completely flat battery and a fully charged one. The peak current was nearly 500A, but after a couple of seconds, it had dropped to below 200A and within 20 sec, to less than 50A.

I also tried a pair of batteries connected through a certain 180A relay powering a stalled Warn X50 4.7Hp motor. The motor drew 560A at stall and the cross flow between the batteries was about 250A. Unfortunately the motor let the smoke out before the relay!

In general use with a pair of winches with X50 motors, even using both hard, the cross flow between the batteries was less than 180A - hense the choice of a 180A relay as opposed to a 500A (which was another, but more expensive option).

Although a pair of XP motors will draw over 1500A at stall - they too will let the smoke out even quicker.

Si

[FridgeFreezer]

a stalled Warn X50 4.7Hp motor. The motor drew 560A at stall .... Although a pair of XP motors will draw over 1500A at stall - they too will let the smoke out even quicker.

Steady on Si - this would suggest someone has measured the current a winch motor draws, and last I heard that was well beyond the means of everyone but NASA