Should I Hook up two 12v gelcells in parallel for car booster box?

Hi -

I've got one of those booster boxes that's used for jump starting cars.  It was powered by one large 12V gelcell, now dead and not chargeable.   I have two 12V gellcels, each of which is much smaller.  A digital VOM reads them at within about .03 volts of one another.   I was thinking of connecting them in parallel.  Aside from the issue of whether the  connectors can handle the amperage, I was trying to think of how this would work out.  Any ideas?  Thanks.

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Adam314Connect With a Mentor Commented:
This is generally not recommended, because as time goes by, the battery voltages will not be identical.  Meaning they won't wear out exactly the same.  They may not even be that close now when under load.  As soon as 1 battery voltage is different from the other, current will start to flow.  This will create a lot of heat in the wires, and the battery.  If to much current flows into a battery, it can become damaged, leak, or start a fire.

With such a low resistance in the wires, it won't take much voltage different to create a lot of current (up to the limit of the battery)
If V = .1 volt, and R=.01 ohm (not uncommon for wire):  I=10 Amps!!
V is the difference between the two.
You have to maintain 12v, so hooking them up in parallel will do that.  If they can't provide enough amperage to start a car, though, it won't do what is intended.
The total current will be  the sum of the current of each battery, btw.
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I doubt the internal resistance of a typical charged gel cell would permit a 10 amp charging current at .1 volt
It can be done but as adam314 said it is not generally recommended for the reasons he gave there. His example may be a bit exagerated, but his recommendation is not.
Hooking gel cells in parallel is mostly okay.  But using them to start a car is not a great idea-- these cells are not designed for that huge current draw over a short time period.  That's probably why the original one failed.
Just get a regular car battery-- it will be cheaper, provide about 10 times the current,, and will last much longer.
lewisoAuthor Commented:
Thanks for this info -

So from what grg99 says, (which makes sense to me) those booster units that come in a plastic case with cables attached (and i think all use gel cells) are not that great of an idea?

Could it be that there are special gel cells designed for this ? - the original one is a Yaohu 6-FM-17 (YH 12V17Ah.20HR).  What is that spec?

One more part of this - there is a rattling sound from this big gel cell when it's shaken.  Significant ?  It does still have some voltage, but only at low amperage.

d-glitchConnect With a Mentor Commented:
12V17Ah.20HR ==>  That spec means 12 Volts (obviously)  
                               and 17 Ampere Hours but only if you spread the discharge over 20 hours

                               To discharge the cell that way means 850 mA per hour for 20 hours
                               The load resistance required is 14.1 ohms.

You can discharge it faster, but you don't get as much energy out.

Car batteries are specified for Ah and for Cold Cranking Amps.

The booster boxes are great.  I think they may have  Capatteries not Gel Cells.
gel cells have a higher internal resistance than your typical car battery.  

A typical gel cell will be in the range of 10 to 30 milliohms

A typical car battery will be in the range of 2 to 4 milliohms.

So in a tug of war the gel cell loses, big time.  Only about 10% of the power is going to flow into the car battery, the other 90% will be dissipated in the gel cell.

The only good news is if the car battery is completely discharged, it's likely to go quite a bit up in resistance, so it will not drag down the gel cell so much.  

Adam314 answered the question asked. I added a very little bit. grg99 added some specific info and d-glitch decoded a model number correctly
I think the fact that they don't provide enough current is the answer to why it's not a good idea.
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