Re:Battery Voltage

Hi,

It’s definitely the alternator or regulator (it’s probably built in to the alternator) which is on the way out.

A lead acid battery is nominally 2 to 2.2v per cell, this gives the normal range of 12 to 13.2v for a car battery. When a battery is charging it will go up to 2.25 to 2.3v per cell or 13.5 to 13.8v. Float charge, when it is fully charged can be a bit higher , 2.35v per cell, or around 14v.

Absolute max voltage is 2.5v per cell or 15v, but generally only an old, tired battery will get up to this.

Without the engine running,terminal voltage will be around 12 to 12.8v, put the lights on and the voltage might dip to 12v, measured on the terminals. The Jimny battery is quite small, so even a good battery might go below 12v, but not by much.

With the engine running, even at idle expect to see 14v with minimal load and still around 13v with lights on.

Low voltages, especially flickering lights means low output from the alternator, just change it and you will be okay. By the way a faulty alternator takes it’s toll on the battery, so best to get it changed quickly since the battery is new.

Thank you all for the help on this matter.

Measured the voltage with RPM 2000-3000 and the voltage was up to 13.4-13.8 without the battery being charged (lights off).

I put the battery on charge which took about 2 hours.

In the meantime, I took out the alternator to have a look. Looked like it was the original - Denso branded (small date of manufacture was made out to be 1998 in Japan) so roughly 21 years old at this point.

Cleaned out the terminals with degreaser and cotton buds with wire brush on the negative terminal and new nuts to replace the slightly corroded ones that came off.

Hooked the alternator up to a drill however it didn't spin it very fast so that was a bit of a flop, but it did at least confirm it was working after tinkering with it.

Alternator back in the car with the battery now fully charged. Hooked everything back up and had 12.8v on the battery.

Turned engine on and got 14.4-14.5v which dipped to 13.6 with lights on high-beam - I thought I had cracked it for a moment. Left on for around 2-3 minutes and the voltage dropped back down into the low 12's and the lights then began to flicker again. Turned lights off and the voltage went back to 12.8v (engine on) however it was dropping slowly. Over 5 minutes it was down to 12.7v .

Given what everyone has said and the results I have found (again, thanks to you all) I've bought a new alternator. Should arrive on Tuesday.

A quick micro update:

I chose a new alternator which outputs 96 amps instead of the standard 75 amps. To compensate, I thought I would check out the main fuse and discovered it was an 80 amp so I took the fuse box apart to get a good look at it and check what type I needed to uprate it.

For future reference, the standard main fuse in the G13B 1999 Jimny is an 80 Amp PAL B Auto Link (Slow Blow) fuse made by Pacific. I am replacing it with a 100 amp equivalent.

Anyhow, off to the side of the main fuse is a small bank of 4x 15amp fuses. This covers the Left/Right headlights amongst other things (e.g. a/c if installed). I previously noted that my lights were flickering sometimes which I believe is primarily down to the under-performing alternator, but now I also believe it is the connector block going into the fuse box for my lights. The connections were extremely corroded (copper/steel) to the point where the wire (white/blue) freely fell apart:

The other two were not much better and with a little flexing, the wires too fell out and were very corroded. The copper connection tabs on the fuse block itself were OK, however, the high-beam contact pad was nearly all oxidised and had turned a green colour - I bet this is part of the problem!

I took apart the connector block after wrestling it out of the fuse box itself and then managed to extract the left-over spade connectors that were stuck inside which were very brittle and oxidised:

(Please ignore my rough and ready hands)

Ordered some spade connections and the new fuse so hopefully, I can get it all back together with the alternator early next week.

This looks like the connections on 20 year old motorbikes.

Maybe it would make sense to check all or most connections under the hood. When I have connections like this, and have good chance of flushing, I use A cleaner for my alloys that says „acid free“ but isn‘t. The cleaner turns from clear to red, if it encounters dirt or corrosion. This removes the oxidation nicely and after 5 min soaking I thoroughly, ver thoroughly flush the parts with water and then let dry.

Before reassembly, I use some corrosion protection fat (Fluid Film Max) from a spray can. With this, so far I had no problems with the electric system.

All the best from sunny Vienna!

New alternator showed up today at about 15:00 and I managed to fit it and replace the headlight connectors in the fuse box by 16:00 and IT WORKED!

12.8 Volts on the main battery (engine off)

14.4 Volts to the main battery (engine on)

14.2 Volts to the main battery (radio/lights + every other power-consuming thing switched on)

I will monitor the voltage on the main battery just to make sure it hasn't been damaged at all by the poorer alternator before and if needs be get a replacement - might be nice considering the other (dual) battery is new, the wiring is new and the alternator is new...would maybe make sense.

Thank you to everyone who helped diagnose the problem - gave me great confidence to get it sorted out.

Edit: the old (new) headlights are back in and working better than they ever have before:

Result. I think your battery will be fine and good to know your Jimny is humming again.

Robin