split charge query with voltage readings

[gti mad man]

ok my brand new lesuire battery went flat while away after about a hour of just running a head unit so pondering on the possibilty that the split charge isnt working correctly i have taken the following voltage readings....

Cranking battery 13.2v idle 13.9v with engine running

lesuire battery 13.1v idle 13.3v with engine running


what do you make of these little low or what youd expect from a 45amp alternator?

[eatcustard]

its that idle as in engine ticking over?

[gti mad man]

yup but if i rev it up reading go up by about 0.1v about it

[eatcustard]

What it like after you turn engine off (after about 10 mins take reading)

[eatcustard]

My starter battery is 12.63v
with engine running its 13.9-14.1v

[gti mad man]

havnt tried after 10 mins just realised your from broomsgrove was up safari park t'other day

[Red Westie]

Bosch set their regulators with a tolerance of: 13.9 - 14.2Volts...both batteries should be reading the same voltage from the alternater as they are directly linked through the split charge relay which begs the question of why the difference.....maybe there are poor connections in the split charge circuit, either across the relay contacts or the spade connections?
Voltage output does seem a little low and a 45amp/hr alternater is a little underpowered for a two battery setup.
Martin

[gti mad man]

cheers can i easily swap for a bigger alternator 1.9 d.g petrol?

hum now question is do i just junk the relay for a fandangle split charge?

b the way thoose reading are with nothing drawing voltage other than the vans fusebox i.e no radio zig lights fan etc

[kevtherev]

cheers can i easily swap for a bigger alternator 1.9 d.g petrol?

hum now question is do i just junk the relay for a fandangle split charge?

b the way thoose reading are with nothing drawing voltage other than the vans fusebox i.e no radio zig lights fan etc

I have had both on Andy 45 and 90 amp with no improvement.

When I fitted a new Bosch regulator/rectifier it improved.
Don't faff about with the split relay it is working well

[gti mad man]

should add im now running a car amp that pulls a bit however this was disconected when the radio is on the lesuire battery

is that the black pack at the rear of the alternator?

[irish.david]

Sounds like the wiring is just getting on a bit. Most people don't appreciate that as wires get older their resistance increases slightly. Add this resistance to any connections that are starting to oxidise and it adds up to a fairly decent sized voltage drop between the alternator and batteries. To see if this is the case you need to measure the voltage at the alternator terminal and compare that to the voltages at both your batteries. As most alternators put out a voltage of 14.4v i can use that number for comparison.

To start with you need to be aware of the relationship between voltage, current and resistance as described in Ohms law. As we want to find a resistance the equation we need is R = V / I where V is voltage, I is current and R is resistance. The voltage drop to the cranking battery with the engine running in your case is 0.5v. With nothing running the electrical system is probably drawing around 2 Amps to power things like the dash and the engine ancillaries. Bearing in mind that this current figure is normally a lot higher just after you started the van while the alternator puts back the juice it took the battery to start the engine so 2 Amps is pretty conservative. This means we get:

R = 0.5 / 2 = 0.25 Ohms

This means that if your wiring increases it's resistance by only 1/4 Ohm your lose 0.5v at the battery under an almost no load situation. With the heater and headlights and stereo on the current could go as high as 40A so with the same 1/4 Ohm resistance as before you'll get:

V = I x R = 40 x 0.25 = 10V

Now of course this doesn't mean that the voltage drop will be 10V in reality as the battery will take most of the load but it does mean the battery will actually be draining when you're driving along with everything going full pelt.

Moral of the story. Clean your connections from the alternator to the battery; this is where the majority of the resistance will be. Run a chunky cable from the alternator output directly to the main 12v feed on the starter motor; the standard wiring goes through the black box at the forward end the the engine bay and more connections mean more resistance. And electrical cables don't last forever.

Dave

[Cafnod]

Fully agree with Kev and dave but I would add that once you have charged any battery and it is drawing no load a 5 watt load (typically interior light) should be switched on after the battery has finished gassing for one minute to bleed off any surface charge in order to obtain an accurate voltage reading.
Please be aware that many cheap multimeters arent wonderfully accurate.