Small(er) Batteries

[Fonzey]

Hi All, wasn't sure which section to post this but I guess home mechanic covers it somewhat.

I'm looking to make some space in my engine bay (2005 Impreza STI) for some other bits and bobs and the battery is a good place for me to start. I can either relocate to the boot, which would be a lot of time/effort or I could just splash out on a new smaller battery.

I've seen the likes of the Odyssey Extreme 25 (PC680) which size-wise would be more than small enough, but I'm not sure whether it would be suitable for my usage of the car. It's a daily driver so has alarm, stereo, amplifier driving two speakers etc - and worse of all my usual mileage is 7miles to work and then 7 miles home...

That seems to be a recipe for running a battery flat, so I'm not sure whether size scales linearly with capacity in terms of car batteries.

Does anybody have any thoughts on such as the Odyssey PC680, or is relocating my only real option?

Ta!

[ch427]

looks too small to me, what is the Ah and cca rating of your existing battery?

[Fonzey]

Honestly, not sure - but when I put my reg into a website battery selector type thing it suggests the following:

CCA: 540
AH: 60

Much higher than the Odyssey - but I've really no idea what those numbers mean.

The Odyssey is way smaller than what I need - I'm looking to roughly half the size of the battery I've currently got so if I can get similar spec from a smaller battery, even if it means paying more - I don't mind.

If I'm asking impossible things from physics and chemistry, it'll be relocation!

[ch427]

cca is cold cranking amps and Ah is ampere hours, im no expert but with uprated audio etc i would expect to see these values matched on the new battery.

[Fonzey]

Thanks

I've done a bit of research on the meaning of those values so I'll certainly try to match them with any replacement battery.

I actually got the recommendation for the Odyssey 680 from another Impreza owner which suggests that the cold crank ability is sufficient, I guess that would either work or it wouldn't regardless of "other" load on the battery, but the AH figure I would imagine is where I'll fall down, with few miles per crank and a heavier load whilst driving.

[ch427]

if its an agm battery they dont take as much abuse either, they dont like short trips etc.

[Mikey G]

ch427 said:

if its an agm battery they dont take as much abuse either, they dont like short trips etc.

I thought that was the complete opposite as AGM's are used in pretty much every vehicle equipped with a stop/start system.

[ch427]

They charge a lot faster which is why i thought they were used in stop/start systems?

[Mikey G]

They charge a lot faster due to the nature of the charging system in stop start vehicles. They have to take more abuse due to the constant stop/start required of the engine. Their life tends to be shorter due to the nature of a stop start vehicle which if used around town a lot can be considerable, but if you were to compare cycles of a standard car to a stop start the AGM will have performed more due to the application it is designed for. If a standard battery was used in a stop start car I bet the life would be so much shorter.

In answer to the original question though, as long as the CCA's are high enough, you can skimp a few AH's for a smaller battery, but remember the lower the AH's the quicker your battery will go flat when you need it.
For what its worth, if you need the space at the front, stick the original sized battery in the back.

[Fonzey]

Cheers Gents, I'm leaning further towards a good battery relocation job. I'm not great with electronics, so this will be one of the few jobs I don't attempt DIY I think.

[ch427]

i killed one in under 2 years as i didnt charge it enough but that was just my stupidity!

[eltax91]

Is your car normally garaged? If so, why not get yourself the smaller one but get a CTEK charger and remotely located plug-in fitted so you can top it up when it's at home?

[Mikey G]

I have a 22ah AGM in my boost pack, it is designed to go to a lower state of charge than a standard battery and be recharged that little bit quicker, works a treat and better than the previous PB battery fitted before it. I do use it nearly every day though for my work... It started a 2.4 transit diesel with a flat battery with ease.

[Fonzey]

Unfortunately no garage, so can't trickle charge.

Mikey do you have any info on that battery? How big is it physically?

[Mikey G]

Its not a battery I would fit to a car i'm afraid, like I said its in my boost pack which is ideal for that purpose. I was just explaining that an AGM battery is more robust than a lead acid.

[Evoluzione]

If you put it in the boot you'll have to use some very large section cable and/or may also end up fitting a bigger battery due to the remote location lowering its effectiveness.

What about fitting a bigger amperage alternator?

[Mikey G]

Evoluzione said:

What about fitting a bigger amperage alternator?

Bit pointless if you have a smaller battery to store that extra energy.

Also a smaller battery will take less time to get to full capacity than an original battery using the standard alternator.

Right, forget your CCA's for a minute and concentrate on AH's
60AH means you can put a load of say 1 Amp on that battery and it will take 60 hours to discharge, or a load of 60 Amps will take it an hour to discharge.
Same goes for a 40AH battery, i.e. a load of 1 Amp will take 40 hours to discharge and a load of 40 amps will take 1 hour, etc...

Now lets look at it the other way, if you have a 120 amp alternator, and we forget the cars electronics running for now, this means if the 60AH battery was flat after starting (big if..) then the 120 amp alternator would charge the battery to its capacity in 30 minutes (120a/60minutes = 2 amps minute), assuming a linear charge. But a 40 AH battery will only take 20 minutes as there is only 40 amps that need to be replenished.
Bit like a fuel tank, the smaller the tank the quicker it empties assuming the same MPG's, but also the less you can put back in when you do stop to refill it..

This is not 100% accurate by the way, there are many other factors involved but it gives you an idea in laymans terms

Edited by Mikey G on Tuesday 12th November 22:11

[ch427]

just out of curiosity what are you fitting that needs the battery to be smaller?

[Fonzey]

It will allow me to reshuffle things a bit in the engine bay and give me options for FMIC piping, Oil Catch Can installation and plumbing and stuff like that.

It's not a big rush, more of an investment if I can clear up that valuable real-estate

I've been going through the specs of various batteries and I think the Odyssey PC950 could be a good compromise between performance and physical size.

[GC8]

I think that youll be alright with a smaller battery. I used one everyday for a year in a highly track focussed car and I didnt have any issues. You need to keep in mind the reduced capacity, of course. I fitted an Anderson jack to allow the addition of a second battery if need be, but I didnt ever have to use it.