Connecting car batteries in parallel?

You better buy them from the same manufacturer as well, because there are slight differences beween producers and types, like different lead alloys, causing slightly different topload voltages, temperature dependencies, ect. Also make sure that cooling air can reach all batteries equally,to avoid unbalanced temperature and charging.

sure it is, as long as all batteries are in good shape how ever, it does pose a problem when one unit decides to have a shorted cell. i have seen fires from this. those that do it correctly normally have a large fuse link (fast blow) for each battery.. and in most cases where a cell goes bad, it will grain the other batteries. scenario is, you end up replacing all batteries at the same time.

I wouldn't use a mess like that without fusing each battery where it connects to the buss bars. I watched two morons barely get out in time when their mini pickup truck went up in a fireball. Their competition stereo system with with a dozen unfused batteries shorted out and set the fiberglassed speaker cabinets on fire. Their reason for no fuses? They loose points in stereo competitions if the judges see fuses.

The 'entertainment' system for Swissair 111 was installed by cowboys, unfused and without the ability of the flight crew to disconnect it. All on board died when it caught fire.

Why would it be a big no no.

You could connect them safely via diodes but without there is some concern about voltage differences and the resulting currents.

No, and neither is being burnt to death.

-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85.

Michael A. Terrell Central Florida

isn't seeing the smoke released part of the competition ? :)

"All batteries are new, of the same model,"

That's the ticket. All of the batteries must (Ideally) be identical.. Same type,size,age,etc.

your batteries much be in happy harmonium balance!

I was assuming safety and other cautions were being observed.

The more you hook in parallel the bigger the mess when one gets a shorted cell, drops 2 volts and sucks all the energy out of the other batteries.

if you isolate the batteries using diodes so that current can't flow from one battery into another there should be no problems as a failed battery cant damage its neighbours.

Bye. Jasen

What effect would a failed battery have on the common charging equipment?

technically there is no problem if all is working as planned... minute differences in voltages soon iron out and when on load make a marginal difference

The problems start when faults arise and good batteries can provide all the power needed to start fires etc when a cell goes short.

A good parallel system should have some circuity to protect against this - at it's most simple, some really beefy diodes hower a better solution is with power MOSFETs so individual batteries can be taken out of circuit on discharge or failure. Charging would require a similar set-up but reversed and watching for excess charge current and general signs of batteries in distress.

depends on how the equipment and charge circuit were designed. If you just have a load of parallel batteries and an alternator connected to their common output as long as everything stays fine, there would be no real consideartion... prob is, things seldom "stay fine" and if you get a short in a cell in one battery, the voltage will drop and the remaining batteries (and alternator if charging) will pour power into that battery... fires start like this. At the least, it will discharge your setup over time. Ammeters (centre zero) in series with each battery output will at least give an indication of what each is doing and so help pinpoint the fault.

Why break your back? just design a nice simple controller (based around a micro-controller witha few a2d inputs) and control the whole caboodle with good thyristors and a display for feedback.

I wouldn't use diodes - for one thing, you've got their forward drop, and for another, you'd need a whole nother set for charging.

I'd just slap some fusible links in there.

Cheers! Rich

Extremely common to parallel 2 to 4 IDENTICAL batteries in marine leasure applications. Fusible links between the batteries and the bus bars are *strongly* reccomended. The batteries should be from the same batch and must be kept as a set. If one fails, you are supposed to replace the *whole* set. High circulating currents are somewhat of as myth as the impedence of a 'flat' lead acid battery is fairly high and surface charge rapidly reduces any initial voltage difference. However as mentioned by others one bad cell will kill the whole set.

Yes. If you look at a lot of vehicles with 'larger' engines, it is common to have the electrics using two 12v batteries, and parallel them during normal running, but series connect them for starting. Rally cars, and things like the Hummer diesel, do this. The norm, is to have heavy duty fusible links, rated just a little above the highest likely starting current, built in close to each battery. Often pigs to get and and replace, when a problem appears, but without them, the wires between the batteries become the 'limiting factor', and can give some 'exciting' effects....

Best Wishes