It's a good sound system!
I've managed to find the specs for the amp online, and it says the current drain, at the rated output, is 24A. That's still pretty power hungry isn't it?

Yep Steve. That may well be your problem. See how the batteries stand up after a few days off the music.

Steve
I asked a friend of mine who's up to speed with 'things electrical' for advice.
It seems like the battery/ies that start your truck are designed to deliver high amperage quickly but do not like going under about half their charge. They should not be deep cycle batteries.

For slower discharging batteries i.e deep cycle ( for fridges etc) these are designed to give less sudden amperage but for longer and don't mind being totally discharged.- and they are more expensive too-

You do not mention how your system is set up but it seems you may be using 3 of the same deep cycle ones and there is probably your problem.
And as for your hifi..... that's very power munching
Sounds like you may need new batteries and of the correct type.

For slower discharging batteries i.e deep cycle ( for fridges etc) these are designed to give less sudden amperage but for longer and don't mind being totally discharged.- and they are more expensive too-

As I understand it you shouldn't be discharging your deep cycle batteries fully if you want them to last. I'm no expert but from what I've read discharging to 2/3 or max 1/2 regularly will ensure a good lifetime for your deep cycles, any more than that and they'll suffer.

There's a piece here with some good info.

Bertrand, I have 2 12V truck starter batteries connected in series to provide 24V for starting the truck, and indeed they are designed to provide a high current for a short period of time. Then I have 3 12V leisure batteries (aka deep-cycle batteries) connected in parallel to provide 330 amp hours for the campervan side of things (ie: fridge, lights, pump, music etc). Deep cycle batteries, as you mentioned, are designed to provide a low current drain over a long period of time, which is what the campervan side of things will do. This is how the system is designed, and indeed should be designed.

As for deep-cycle batteries being happy with being completely discharged, this is incorrect. You will severely shorten your batteries lifetime if you completely discharge them. An example comparison of discharge depth versus life in cycles (a cycle being one charge/discharge cycle) for one type of deep-cycle battery is: 30% ~ 800, 50% ~ 400, 80% ~ 250, 100% ~ 100.

As for the amp, I've been doing some thinking on that: It would be interesting to know what current drain the amp causes in practice, coz we hardly ever listen to the music at full blast. I'd guess a far lower current drain, proportional to the percentage of max output we listen at?

My reasoning is this: I completed the entirety of the west coast of Africa (which is not at all set up for tourism, so there was practically never a chance to hook up to mains) with the 3 batteries I currently have. At one point, in Congo-Brazzaville, we got stuck for 2 weeks fighting with the Angolan embassy to get visas for Cabinda. During those 2 weeks we sat virtually all day every day in the truck, waiting for word from the Angolans, and the South African embassy who took up the fight for us (to no avail, but that's a long story... if you're interested, read here: OverAfrica - Congo-Brazzaville - the exodus)

Anyway, as you can imagine, we spent a lot of time listening to music while we twiddled our thumbs (probably in the order of 5 or 6 hours a day I'd guess.) There was a fair amount of rain around, being at the equator and just before the onset of the rainy season, so the solar panels weren't producing a huge amount of charge. At no point did our battery charge die. The solar panels were capable of keeping up with the current drain.

With that in mind, I'd say that perhaps the way we use the sound system is ok, and it's one or more batteries that have died. It would also probably be worthwhile getting a higher-current rated 24/12V converter for the truck's alternator, but not hugely necessary.

Any thoughts?

Thanks
Steve
OverAfrica - overland, over Africa

Steve- super website- enjoyed reading it- thank you

From what you say above I'm scratching my head here....all I can think of is that it's perhaps just time to replace your batteries with new ones - if so I would personally chose the same type, brand etc and replace them all together.
Other option as you say is going the higher-current rated 24/12V converter for the truck's alternator.
The minimalist in me also says to lose the hifi..... and go the Ipod and 'inMotion' IM9 route!
It has a mains powered transformer but can run off 'C' batteries or better still on universal adapter so no need for mains- unit only needs 17 volts and 2.3Amps- way (too) loud, awesome sound !

Try and get in touch with Quintin who has a somewhat large truck! he might be able to go into details of how he has sorted his out.

I've been speaking to Collyn who wrote the Motorhome Electrics book (which I ingeniously left behind when we started the trip!)

His advice for checking the state of the individual batteries is this:
Fully charge the 3 batteries. Isolate and measure their individual voltage levels. After 30 mins re-measure their individual voltage levels. If there is a significant drop in voltage in one of the batteries (+0.2V) this is most likely the culprit. Now connect a load that draws about 1`0% of the battery Ah capacity (if 100 Ah use about 10 amps) to each battery in turn, and measure the voltage with the load connected after at least 2 hours. Batteries should really be within about 0.2V - if any battery has a voltage 0.5V lower than the others, that is the one that is stuffed.

I'm planning on doing this today... I'll report back with the results! Here's hoping I actually get results that make sense!