Bongo Fury

For a few reasons I was suspicious that the voltage was a bit low, nothing drastic affecting instrumentation or the hold light. It is possible that my alternator has had a slight roasting with a blowing exhaust manifold. My LB is a 3yr old 60 Ah Halfords job so could be on the way out. My SB is Bosch 334 less than a year old so should be good.

Set up is simple. No EHU. Merely is a Willington charging the LB. Usual 4 circuits transferred to LB.

I did some tests and would welcome some help in interpreting the results.

Battery volts


Alternator Test

• LB disconnected
• Aircon OFF

(hope the table makes sense, voltage measured from the starred * point to the one with the voltage recorded)


Should I be worried about the decreasing battery voltage during the load test?
To my untrained eye, everything else is looking ok ish and perhaps I just need a new LB !

i dont see owt wrong with the results at all, but my middle name isnt geoff. .

the only thing i would say is the leisure battery is a little small, 60 amp and 3 years of age, it poss wont last much longer, most run far larger than that.

Nothing jumps out as being wrong at all, the loaded voltage test is fine as well, the exact RPM will vary the voltage also as the battery charges it will vary.

The LB at 60AmpHr and three years old could be coming towards the end of its life if its been well used, but that's not based on the tests, simply age and capacity, best way to check is to record its capacity simply giving it a full charge and then turning on a couple of lamps so that you draw ideally 2 Amps (or something known/measured in that range) then leave that on until the battery voltage falls to about 11 volts noting the time it takes to do that so if it was actually 1.5 Amps load for say 40 hours then the capacity would be 1.5amps x 40hours = 60AmpHours capacity.

Cool. Thanks guys. It was pleasing to see the volt drops all adding up about right.

Didn't want to jump to conclusions about alternator or battery. I am still going to check the charging voltage after driving a while because I think this is in 13v range but need to be clear and confirm this now I know the voltage drops are fine.

I highlighted the declining battery voltage during the load test because if that decline continued over a long drive the batteries would only be charged up to a maintenance level and not 100%. I'll check.

All the time that the battery voltage is above 14 volts with the alternator spinning then it wouldn't concern me, if it fell below 14 volts then that would indicate that something was less than happy. My first call then would be to check that all the connections - Alternator +ve, 100Amp fuse, battery terminals, earth connections at battery and alternator were all good - the 0.22 / 0.23 volt drop between Alt+ve and Battery +ve is not excessive but would make me look slightly closer at those connections cleaning and tightening them to see if it could be improved, but as I say is not excessive if the battery was charging at 10 Amps then that indicates only 0.023Ω which is good!

Just been for 2 X 1hr motorway trips today. Batteries started off as left following earlier tests so should have been well charged. Volts at 2k rpm at alternator were 14.1v and at batteries 13.9v. At idle alternator volts dropped to 13.9v

So voltage drops between battery and alternator are quite consistent and alternator volts not exactly fantastic.

I'll try to track down volt drop on +ve as suggested above as there shouldn't have been a high load / volt drop when i took these readings - side lights only.

Took your advice Geoff and swapped out main 100A fuse ( I mangled it, didn't realise it was bolted in ), cleaned all terminals and connectors in the positive battery assembly and ....14.47v across battery terminals when idling and same when loaded at 1800rpm.

That's a significant improvement. Thanks.

Yup its bolted in! I do emphasis this on the web page in all the places its mentioned! Its easily done as none of the others are and its not obvious from above see http://www.g8dhe.net/bwm/ and look under all the fuse Tabs!

g8dhe wrote:Yup its bolted in! I do emphasis this on the web page in all the places its mentioned! Its easily done as none of the others are and its not obvious from above see http://www.g8dhe.net/bwm/ and look under all the fuse Tabs!

oops. I didn't look on any web pages. Well here it is again in case anybody finds this thread and wonders how difficult could taking out a fuse be ?

cmm303 wrote:Took your advice Geoff and swapped out main 100A fuse ( I mangled it, didn't realise it was bolted in ), cleaned all terminals and connectors in the positive battery assembly and ....14.47v across battery terminals when idling and same when loaded at 1800rpm.

That's a significant improvement. Thanks.

BUT ..... this voltage drops by over 0.5v when warmed up.

Last night immediately at end of a 25 minute run: 13.81v - 13.88v (idling-revved).
This morning started it up and it read 14.45-14.48v (idling-revved).

Alternator trouble?

With a Scotland tour looming in less than 2 weeks this behaviour worries me.
If a new alternator is needed, recommendations for where to source would be grateful as they are not cheap and I've seen issues here with people buying underrated ones.
I've also seen adverts from companies reconditioning alternators, unfortunately the nearest I've seen is well over an hours drive each way.

Battery voltage & capacity is dependent on temperature, this is one of those things that makes it very difficult to say what state a battery is in when you only measure the voltage alone!

Because of this the voltage either floating or on charge will vary dramatically - DON'T READ TOO MUCH INTO BATTERY VOLTAGE ALONE !

I was worrying more about the alternator but taking the hint I'll move on and finish those curtains rails for the back

The only certain way to check the alternator is to measure the charging current using a clamp meter, measuring the voltage will show up gross faults, but often will give unclear readings because its the battery/alternator combination that sets the voltage not the alternator itself.

Geoff, thanks for all the info and interesting graphs. Don't have a current clamp. Decided going to replace the 3yr old LB and see where that gets me.

If your up at the National Bash come and seem me, I carry a clamp meter in the van for just this purpose!