AGM Charge Voltage

**Konradical** · 28th December 2021, 01:43 PM

Hoping for advice..

Earlier this year I replaced the starter battery in my 89 RRC with a Optima Redtop 8002-250. I acquired a couple of them and with the then current battery on its last legs, I thought "why not, it fits".

Over the period when I was driving the car (on historic rego) everything performed great, actually one of the best mods I have done to be fair.

When I returned home form spending the better part of five months in QLD, and eventually SA getting out of lock downs, etc.. I took the ol girl out for a drive. While I was away, neg terminals were all off and was pretty well set up for storage. She started first go, albeit and bit slower than normal, but all was fine. That was until one evening I went for a drive and it developed a miss at idle. I was sure I was done with the miss after a lead and plug change post cam change.

One of the tests I did was to the battery. The little battery tester I have said that the battery was fine, but needed a charge. So I charged it with my Ctek charger on AGM mode. Sure enough, the next day she starts without a hiccup and not a miss to be heard.

Fast forward to last month (November) when I last used the Rangie to attend a Land Rover Club meeting, and it developed the miss at idle again on the way to and remained on the way home (about 50km round trip). Today I started her and while she started straight away, it was a bit slow.

Now with the understanding that anything using power in the vehicle while parked will take from the battery leading it towards not being fully charged, but the month of November had me using her on average twice a week on trips to work (30kmish away) and my mothers place (45kmish) as well as spending my birthday cruising around with the kids for about 120 odd km. This was a mix of traffic and motorway and I have never had trouble with a slow battery after a simple drive in this mix before.

So.. after today's start, it got me thinking and hopefully someone smarter than me can confirm, that the alternator isn't putting out enough volts to charge the battery properly and its only after having a decent charge, that the battery is at its best. I do have a 140amp Bosch alternator from a VW Golf, but it only ramps as high as 14.4V. to continue running this battery, is it possible I need to find something that will push to the 15V or so that these AGM batteries need? Or can I mod the one I have?

Look forward to your answers!

**AK83** · 28th December 2021, 02:40 PM

The battery doesn't specifically 'need' 15v.
If you check the specs for the battery, anything from 13.3 - 15.0v is fine for charging the redtops. (according to optima's specs for it).

more importantly tho(for your info) is what optima recommend for the float charge .. which is 13.2-13.8v ... ie. not too much once it's charged.

Do you have a volt meter fitted in the car? Handy to have to watch the batterys' charge at any point in time.

I have a redtop as the starter in my TD5, and it half died whilst the D2 was still my brothers vehicle, and he didn't use it for a couple of months. No charge for battery .. etc. just sat out the back and batteries suffered because of it.
Now the redtop doesn't hold charge for more than about 3-4 days, unless trickle charged.
I also have a yellow top which wasn't as badly affected.

One of the things I attempted a couple of times now was to AGM+ charge it. Some battery charges have this as a setting on them. Helped to restore some lost ability on my redtop.
I've had my D2 in the shed for the past couple of weeks, trying to get some minor stuff done. So it's sits unused. I've deliberately left if off charge for a short time(say 2-3 days) to watch how low the batteries drop too.
Before AGM+ charge sessions, redtop would easily drop to 11.9v in say 3 days. Yellow top held closer to 12.3-12.4(ish). It obviously depended a little on ambient.

Redtop is fine to charge all the way up to 15.6v(according to specs) .. but the caveat is no more than 50°C battery temp. ie. don't do a 15.5v AGM+ charge cycle in 40°C ambient temps!.
I gave my redtop two maybe three hits for a couple of days continuously whilst the weather over here in Melb was in the low teen ranges(and lots of rain with it).
So far it seems to have helped a little. Before where the battery would settle at 11.9v after say 2-3 days, now I see 12.1v.

So from what I'm understanding: your golf powered charge system seems to be fine for the redtop in normal circumstances. 14.4v at the start of the charge cycle is fine for a redtop.
You don't really need more, if you use the redtop battery spec sheets.
As the battery charges up, voltage should slowly drop, and once charged, hopefully the golf alternator is then charging the battery at the recommended 13.8v max limit.

I have a half hour drive to work: on start up(always in the dark), even with lights on, I see 14.3v on the volt meter(both the USB port+ voltmeter, and the nanocom) .. by the time I'm at work, sometimes just after first light .. so maybe lights now off, I've noticed voltages now at the 14.0v range .. sometimes 13.9, sometimes 14.1

One thing about the redtop tho, is that even with only 12.1v, it cranks the TD5 like it's a sub one litre low compression petrol engine. Crazy fast cranking .. this is relative to the hopeless Century I have in my tdi D1. Same deal where I have a couple of USB/voltmeters fitted to each battery(plus a gauge cluster with volt readout for both batteries). Century shows 12.4v, but sometimes the cranking speed sounds like a tired sloth with sore arms down there turning the crank directly .. woeful!

What model C-Tek charger and check to see if it has this AGM+ charge feature. Advice has been given to use it say once a month or so just to give the battery a small kick occasionally.

**Tombie** · 28th December 2021, 04:14 PM

Would almost put money on it the Krap-tima has started degrading.

They aren’t anywhere near as good as they once were.

**drivesafe** · 28th December 2021, 05:02 PM

Hi Folks and you MUST check the specs for your specific BRAND and MODEL of battery with that battery’s manufacture.

Optimas can safely be charged at up to 15.0v.

BUT most other AGMs MUST NOT be charged with anything above 14.7v.

Hi Konradical, your batteries sound like they are sulphated and some good rejuvenating maintenance will bring them back to a health state.

Follow the link below and then scroll down the page, to the post on Maintaining Batteries.

This will give you all you need to do to recover your batteries.

https://www.aulro.com/afvb/the-verandah/246755-line-auto-electrical-info-8.html

**Konradical** · 3rd January 2022, 10:09 PM

Thanks for your replies and sorry for the delay in getting back on to read them.

I do have a volt meter in the vehicle (Redarc) which also has the Amp reading also - unfortunately the Hall Effect sensor is dusted from the heat. The gauge will read about 14V while driving and doesnt really deviate away from that number. I should probably double check it with a meter to ensure it is accurate.

The Ctek I have is the MXS 7.0, has AGM mode and a recovery function. I have used the recovery mode before on another battery, so know it works. In this instant though, I might follow the thread instructions that Tim has pointed to. After a week of this, I will throw a set of solar panels at it while I am not using it, to keep it up the spec.

I will also pull the Yellow top out of the D4 and let the solar panels go to work and get it back up to spec, it seems to have lost a bit of capacity recently.

Thanks again.

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