Battery Drain / Helper Batteries

[smalldriver]

Hi guys! I have a small race car, magneto driven (so no electricity is used by the engine once the car is started - I can disconnect the battery and the engine keeps running). My question is "Why does a large helper battery, connected in parallel, provide little or no assistance to the small on-board battery?"

Details. I have a small super-efficient radiator fan, electric fuel pump to supplement the mechanical one, and extensive data acqusition on board the car. Engine aside, nothing else electrical (as in no lights, no powered dash instruments DA aside, etc.). Feeling brave, I used my medium quality ammeter to measure the draw of each of these items - ammeter between battery positive and wires on battery). 4.2 amps total draw. An event involves approximately 20 minutes of running, 6-8 starts, fan (biggest draw aside from starts) only on for 4-5 minutes. No alternator.

So I bought a 5 Ah battery, acid mat. Car only started twice, battery started out at 12.5 v or so, for two events. I replaced the battery between the two events. BUT. I always had the big car battery, one of the red top gel ones, hooked to the car when in grid, and when trying to start. I discovered that if the little on-board battery went below 11.2 volts, the car would not start. Leaving the two batteries connected did little to charge the small battery, my feeling is that natural recovery accounted for most of the gain. I also feel that all of the starting juice was coming from the little battery.

Experience with my current, 13Ah battery shows the same. (Though it makes it through an event until recently, battery dying of abuse)

So, how can one car jump another with a dead battery, and this not work on my race car? When plugged in, positive to positive and negative to negative. Ohmmeter shows the connection between the two batteries is good.

A few pounds and/or a HP or so makes a big difference in my class.

Thanks!

Chris

 

[Comcokid]

What you can get out of a battery is not solely dependent upon the amp-hour rating of the battery. The smaller the battery, the higher it's internal impedance (or resistance - engineering term) that is, the lower (what is advertised as )it's cranking amperage.

When you crank the vehicle, the starter needs lots of current. The ability to supply current is more important than the actual amp-hour rating for this brief moment.

Now, when you use cables to connect the "big car battery" for starting, the resistance of the cables to the bigger battery, from a heavy current standpoint, makes getting current from the big battery as difficult (if not more) than getting current from the small battery. In other words, when cranking, the cables and connections to the big battery keep it from supplying much current, so most of your starting current is still comming from the small battery.

You need good cables, and good connections to the external battery. Cables and connections that have as little voltage drop as possible when cranking. The closer to the starter/solenoid these temporary cables attach, the better.

 

[smalldriver]

Thanks, Comcokid. That makes a lot of sense. I (used to) run thick starting wires from the small on-board battery to the starter. Wires from the helper connection to the on-board battery were 10 guage.

During a recent weight reduction program, I changed all these key wires to 10 gauge. Small starter motor, 500cc engine, no wires get hot (during long starter cranking tests). . .

So I suppose that I may have solved/reduced the one problem - helper not helping - and created another: dying battery having more difficulty turning engine over.

Eh, solution will be new on-board battery, same size, I rekon.

Thanks!

Chris

 

[WGJ]

What are the actual resistances that you measured of the connecting leads you used between the 5Ahr and bigger slave battery?

To get any 'charge' into a battery, you need to have a reasonable potential difference between the charging source and the battery under charge, no matter how much current the charging source is capable of supplying. If the difference is small, the charge current will also be small.

Your mains battery charger should be (relatively) constant voltage (say about 16 volts) with a current that varies, depending on the state of charge of the battery - dictated by its terminal voltage.
Most domestic chargers only put out a max of about 6A for a 12V battery. The 16V-12V difference gets you the 6A charge current.

The slave will get you a start, providing the leads are as short as possible and thick.

On a road car with a 'cold' petrol engine, at 20ºC, you could pull 150A to start from a 50Ahr battery, so starting your race car from a 5Ahr battery is obviously going to punish it. It's like a dead short.


Bill

 

[smalldriver]

I have not measured that, Bill (lead resistance). I will next week (car isn't here). I am certain that the resistance was higher from helper battery to on-board than from on-board to starter, Should be nearly the same, now. (recent change)

About charging voltages, though. . . My moderately high quality (home) heavy duty charger, set to charge at up to 15 amps, only pushes a bit less than 14 volts charging voltage (voltmeter on battery terminals).

My small at-event DC-DC charger, that I use between runs, can push up to 5 amps, but it (indicates that it) charges a bit less than 14 volts as well. The folks who sold me the small 13 Ah acid mat battery said that I could run almost unlimited amps into it to charge, but for a decent lifespan it needs over 15 volts to charge.

The best DC-DC charger I could find retail was a fairly fancy/expensive one for R/C aircraft, but I can't change charging voltage. If I could find a DC-DC charger that could push 15-20 amps at higher voltage, I could prob go back to the smaller battery, with thicker wires. Five pound savings - a lot for me! <g>

Too many bad rumors about Lithium batteries to run that risk. . . Battery temps can get quite high at times.

BTW, the run from battery to starter is about a foot and a half. Six inches to frame ground (engine solidly connected metal-metal to frame).

 

[Fabrico]

Normal jump starting of a car is greatly enhanced if the doner car is running and the alternator can do some charging of the dead/low battery. A good portion of the boost can come directly from the alternator.

Generally speaking, hooking one battery to another does nothing. One won't charge the other.

Except for finding shorted cells, measuring voltage on a battery that is not connected to anything is meaningless.

Matched batteries connected in parallel, can survive with very little drain. Mis-matched batteries will usually kill each other.

Charging with over 15 volts will not lengthen the life of a battery, it will shorten it.

As far as how much current goes where in a car, the flow of amps is completely contolled by voltage.

It sounds like you are doing everything but charging anything.

 

[BrianR]

smalldriver, if you are going to measure lead resistance then using an ohmmeter is unlikely to give useful results. If possible measure voltage drop while a certain current is flowing from the battery. Sounds the same but doing it this way eliminates a lot of variables and covers the resistance of terminals and crimp connections as well. eg use a 50W lightglobe connected to the end of the leads driven off the battery and measure the voltage across the globe and the voltage across the battery. Now you have a good feel for the voltage drop.

A way to get very low internal resistance from a small battery is to use a NiCd battery. They have very low internal resistance but you'll need 10 cells to simulate a 12V pack and as their voltage is not quite the same as a lead acid battery you cannot run them in parallel.

 

[Fabrico]

If you can put a helper battery together in a "by-the-cell" fashion such as mentioned above, you may be able to do some charging. At 11 cells you could have 13.2 volts, and 12 cells 14.4 volts. You may need a rheostat or something at times to keep things from overheating, but it would provide charge to a 12 volt battery.