9 replies to this topic

[Gaz66]

Thinking ahead....
I want to get away with the smallest battery for my build.
Ive worked out that with everything "on" the drain will be 650W/54A
The engine is A series 12.5cr
Is it more important to have an alternator that puts out 54A than a battery that will as that size battery will be huge?
Or am I not thinking this through correctly?

[68+86auto]

Why do you want a small battery?

 

Unless you are racing, I wouldn't do it. If anything happens and you need to crank more than normal, you need extra capacity. A small battery can easily result in a voltage too low on cranking, especially with a cold engine in cold weather. No problem for racing as you would probably have a jump starter with you.

[Ethel]

You want an alternator big enough to run everything. but it still won't below about 2000rpm. The most important thing about the battery is the CCA, the amperage it has when the starter is cranking. 12.5 is unusually high CR, unless it has a race cam which will drop the effective compression especially at cranking rpm.

 

Having enough battery power to get you to safety if the alternator cut out (snapped fan belt) would be worthwhile .

[imack]

I'm running a Powerlite PS20 on my road mini, it's been on there for about 7 years now.
It cranks a 1380 with 11.5:1 cr with an sw23 cam with no issues coupled to a slimline powerlite starter.

[beardylondon]

I was advised many years ago that an 075 size is a good modern upgrade and fits in the battery box, and I have been running various 075 batteries in my Mk1 for 25+ years. OK its not small, but good cranking, as said above!

 

This is the one I currently have, no complaints:

 

https://www.tayna.co...es/exide/ea612/

 

 

try discount code: SRUKFIVE

(saw it on a youtube channel I watch)

Edited by beardylondon, 26 May 2026 - 09:24 AM.

[Gaz66]

I'm running a Powerlite PS20 on my road mini, it's been on there for about 7 years now.
It cranks a 1380 with 11.5:1 cr with an sw23 cam with no issues coupled to a slimline powerlite starter.

I think that's the sort of thing Im looking for imack and nice to see youve given it a good testing 👍
It HAS to go behind the drivers or passenger seat (no room or box in boot) so physical size is critical too.
The seat pan has been removed and the seats are well back.
I havent done a thorough search for that battery yet but was under the impression the lithium ion batteries couldnt be charged with an alternator, or is that just lithium?

[stuart bowes]

Varley make red top batteries which are pretty well-known in the racing circles, decent sizes for this sort of thing (not the usual massive breeze block)

 

https://varleyredtop.com/batteries

 

couldn't tell you which has the requisite CCA and Ah's though (if any) it's possible these are mostly designed for people who jump-pack them to start and then the battery is just there to keep it going (or in some cases, don't even have a starter)

 

kinda depends how much money you're willing to throw at it I guess, if you urgently to lose a couple of 1/10ths then it's worth it

Edited by stuart bowes, 26 May 2026 - 10:46 AM.

[imack]

I'm running a Powerlite PS20 on my road mini, it's been on there for about 7 years now.
It cranks a 1380 with 11.5:1 cr with an sw23 cam with no issues coupled to a slimline powerlite starter.

I think that's the sort of thing Im looking for imack and nice to see youve given it a good testing 👍
It HAS to go behind the drivers or passenger seat (no room or box in boot) so physical size is critical too.
The seat pan has been removed and the seats are well back.
I havent done a thorough search for that battery yet but was under the impression the lithium ion batteries couldnt be charged with an alternator, or is that just lithium?

I've done nothing to the charging system other than fit the powerlite battery.
We also fit noco NLP20 lithium batteries to race cars at work with no other modifications.

Edited by imack, 26 May 2026 - 11:48 AM.

[Gaz66]

I'm running a Powerlite PS20 on my road mini, it's been on there for about 7 years now.
It cranks a 1380 with 11.5:1 cr with an sw23 cam with no issues coupled to a slimline powerlite starter.

I think that's the sort of thing Im looking for imack and nice to see youve given it a good testing 👍
It HAS to go behind the drivers or passenger seat (no room or box in boot) so physical size is critical too.
The seat pan has been removed and the seats are well back.
I havent done a thorough search for that battery yet but was under the impression the lithium ion batteries couldnt be charged with an alternator, or is that just lithium?

I've done nothing to the charging system other than fit the powerlite battery.
We also fit noco NLP20 lithium batteries to race cars at work with no other modifications.

Interesting.....
The lithium batteries sold by demon tweeks all state to be charged with the supplied charger only. 🤔
The minefield starts again! 😁

[stuart bowes]

Id avoid it peronally, SLA batteries have a higher internal resistance so they can only take a certain current

 

Li-Ion (like PS20) and Li-Po's can take a lot more current and possibly damage alternators.  newer cars have smarter charging systems obviuously so that doesn't happen

 

it's 'probably fine' but you can add a unit to make it safer 

Edited by stuart bowes, 26 May 2026 - 01:58 PM.