voltage and revs dropping when using more electricals

[Junglist]

I thought this was usual but reading a few posts on here I'm getting the impression it's an alternator problem. Am I right? This morning I removed my two linked batteries from my engine bay which I had installed in sequence to increase capacity (nothing fancy, no split charging or anything). With them removed, I installed a big old chunky 90Ah battery (number250) with 720 cold cranking amps. I didn't specifically choose it for the spec, I had it at work and thought it must be a better bet than what I had. Was a squeeze to fit it under the bonnet!

I had tested the two that were in before at less than 12 volts at rest. This one's 12.8 to 13 volts at rest 13.9 running, but the more stuff I switch on with the engine running, the more it drops (as low as 13.4). The revs drop also (manual choke by the way so no self adjusting) by a good 200 to 300 rpm if I go from everything off to suddenly trousers stereo on at full blast and all lights on.

What can anyone tell me about my situation and do I need to do anything? Cheers.

[go-for-it1]

Sounds ok to me.........unless you are planning to use it like the De-Lorean and go back to the future!!!!!!!

[Junglist]

so it's supposed to drop then? Got the impression it shouldn't. OK cheers. Can someone else back that up so I can be certain? Thanks.

[Nate]

you should be seeing around about 12V with the engine off. this should then be around 13.5 - 14V with the engine running. so long as it doesn't drop below 13V with everything switched on, it's not a problem.

was slightly concerned when you said "two linked batteries from my engine bay which I had installed in sequence to increase capacity"

what did you mean by "in sequence"?

if you had them in series then you'd end up with 24V which is going to break stuff. if you were having problems with the battery not holding it's charge then just 1 battery would have been the way forward. fitting 2 batteries in parallel would still give you 12V, but if they are not the same capacity and power (i.e. you've got a 60Ah max 200A and a 100Ah max 500A) then the larger one is going to do all the starting anyway, and constantly be trying to charge the smaller one, so it will actually worsen the situation, not improve it.

[Junglist]

I meant in parallel. Not for the extra starting power, but so there was more room to store energy so that the stereo didn't find itself short on power thus losing sound quality. I did it in an attempt to eliminate the need for a capacitor. When I took the readings it seemed it wasn't right, presumably because of the reasons you just stated - making it worse not better.

What I want to know is - have I made a good decision getting rid and putting this big 250 battery in rather than an 063, and should the voltage and revs be dropping when I turn the stereo and/or lights on? The alternator I'm using is a 90 amp one.

Cheers.

[Nate]

when you turn on electrical stuff, you put a bigger load on the alternator and make it harder to turn, which will in turn make the engine have to work harder. as you are not altering the throttle position it is logical that the revs will drop.

as for the battery, a larger battery is probably wise, but if you go too big, it will put a large drain on the alternator, making it harder to turn etc...

i would suggest that if it is only just fitting into the battery tray as you mentioned, it is possibly too big.

[Junglist]

Too big how? What negative effects will it have?

[Nate]

What negative effects will it have?

if you go too big, it will put a large drain on the alternator, making it harder to turn etc...

063 is listed as 43Ah. personally i would fit an 065 at 55Ah or an 067 at 72Ah

[Junglist]

Sorry if i sound thick but what happens if it makes the alternator harder to turn? I'd just do as you say and buy a lower Ah battery but i can't really afford to spend much money at the minute, that's why i'm asking for all the ins and outs. I want to know what problems the alternator being harder to turn will cause. Cheers.

[Nate]

the engine has to turn the alternator. the more drain you put on the alternator, the more drain it will put on the engine. this in turn will make you use more fuel (admittedly quite a small amount more, but definately more)

depending how oversize the battery is, it may reduce alternator lifespan


as an aside: when working on a wind turbine, a dead short is connected across it to stop it completely

[Junglist]

I can stand to use a touch more fuel and possibly wear out the alternator. I've got a spare. As long as the stereo's got a constant supply of good voltage and won't be left hungry i'm happy for now til i've got a bit more money. Cheers for all the advice. A possible future inconvenience is worth it to save money in the short term and maintain the sound quality.