24v battery system question.

[dave pinchin]

Hi guys,
I'm a newby to this site. I have just graduated from a Nissan Safari LWB hightop. Lovely bus but highly prized by thieves. I'm looking at buying a VX (~1996) to replace it.
All good so far.
Here's the techy bits.
Safari had 24v and the whole car ran off 24v. Any 12v was electronically jiggled from this 24v supply.
Had a look at a VX today and noticed that there was quite a loom taken off the midpoint of the batteries.
I always understood this was bad practice because one battery was drained a bit, the alternator then pumped up the system and therefore overcharged the upper battery, which in turn then stuffs the lower battery.
There are ways of electronically balancing the batteries. Does Toyota do this?
Is the 12V loom that I saw an add-on from radio / alarm install?
Thanks
Dave

 

[StarCruiser]

Hi Dave and welcome. The 80 series VX you speak of runs a 12v system for all but the starter. It has a series/parallel start relay that places the batteries in series for starting and parallel at all other times. It's a good system which gives little trouble but some folks convert to 12v and utilise the space taken up by the second battery for a leisure battery.

 

[dave pinchin]

Thanks StarCruiser.
That sounds like a very stable, sensible system. PLUS you can put in off-the-shelf alarm units.
24V alarms are rediculous price. (And hence our losing our Nissan).

Dave

 

[Chris]

Just adding a bit to this, in truth the Toyota system doesn't balance the batteries. One battery is fed directly by the alternator and the second is slaved if you will from the first battery's terminal. Because there are resistance losses in the coupling cable, the second battery never actually gets exactly the same as the first. OK it's small but you asked if there was some mechanism by which the batteries were balanced. No. There isn't. Does that matter? I really don't know. I do know that over time many 80 owners end up with a mish-mash of batteries under the bonnet so even if there was some sort of system, it would probably rely on having two identical batteries. I'm one of those who has converted their starter to 12 and junked the whole 24v system. I saw it as an additional complication that isn't needed. There are probably more 'my 80 won't start' threads posted on here than any other common problem with this fantastic vehicle and many of them are traced back to battery issues relating to the 24v set up.

 

[dave pinchin]

Hi Chris,
Thanks for the extra info.
Just to clarify, In this case the connecting the batteries together effectively balances them. There does seem to be a hang of a lot of extra contacts and cables to go wrong and at crank currents it takes very little to lose all the oomph gained by throwing an extra battery at it.
In the case of my old Nissan, the previous owner had wired a trailer light coupling to the mid point of the batteries.
What this did was
1/ on a long night towing the lower battery was loaded down to 13.9V while the upper sat at 14.4 (max for a good battery at normal temp).
2/ the alternator saw 13.9+14.4=28.3 (0.5 v below full charge) so it poked out more juice.
3/ This overcharged the upper battery and made it hot.
4/ As a battery heats up it's full-charge voltage drops!.
5/ More from the alternator.
6/Bottom battery reaches full charge but top battery is now hot (and therefore low voltage).
7 / Alternator pokes out even more juice.
I guess you can see where this is going. A trip down to the battery shop.
I am sooo glad Toyota have put a 12V system underlying their 24 and have some way of keeping the batteries sane.
Another advantage of the Toyota way is that it is feasible to replace only one battery if only one fails.
In the Nissan it was important to do both.
Thanks all for a lively discussion.
Dave

 

[Towpack]

Opinions differ on the reliability of the 12/24v system on the 80 series. I've heard it said that it was part of a 'cold weather pack' used on vehicles for certain markets as not all versions had it. Maybe. All I know is I've had no issues with mine in the last 13 years of ownership. I did have to replace the 2 batteries at 9 years. They had been quite deeply discharged on a couple of occasions due to a parasitic drain fault when parked up. They may have lasted even longer if not for this. When they were tested prior to replacement their capacity was well down but there was not a huge mismatch. Personally, I don't buy the imbalance charge issue. In theory, yes but the cable linking the 2 batteries is big enough to take cranking current so losses across this 3-4ft length will be negligible IMO, as long as all the connections are sound. No one can argue that getting rid of all the 24v switchover gubbins removes some complexity from the starting system but unreliable complexity it is not. JMO

 

[Chris]

Sorry TP I meant that there are losses on the cross link cable when charging. Not cranking. I'm just pointing out that my understanding of balancing batteries is that every battery in a bank gets treated equally and in effect is one big cell if you like. This requires some clever stuff normally. The batteries in the Cruiser are NOT balanced, they're simply connected. With one battery sitting at say 14v and one sitting at say 12v, when you connect them the car sees 13v. That's not balanced.

If you were to put say an Optima in one side and a standard Varta in the other side, the system would not detect that at all and make any compensation. One battery may require a different charge but effectively the alternator would only give an 'average' across the two. Maybe I'm over-answering the question. It depends what you mean by balance I guess. Typically when one battery goes on this system or the double battery Colorado set up, it takes the other battery down with it and you end up replacing both

Many many people have had no issues at all with the 24v system. I just didn't fancy it in the middle of the desert. It's worth pointing out of course that when the 100 series came along with a 4.2 lump, they went to a single battery ....

 

[Towpack]

Mixing of 2 battery types or maybe even different brands of the same type is definitely not a good idea or recommended. My point about the link cable is that it is of sufficient size to easily handle cranking current so drawing a charge current of, say, 20A max down it is nothing. If the cable was maybe a quarter of the size and 3 times longer then the voltage drop would certainly cause an issue with charging imbalance. The voltage drop on a cable is related to the load/current draw which, on the link cable is so small at charge current as to be negligeable IMO. Batteries do go faulty for various reasons. You could buy 2 identical batteries and one could fail prematurely due to an internal cell failure, then you would have the situation where the dud battery drags charge from the good one. From this perspective I guess converting to 12v removes the possibility of starting problems down to this issue assuming the 2nd battery isn't permanently wired in parallel but I think it's unfair blaming premature battery failure on the charging characteristics of the 24v starting setup.

 

[Chris]

Sorry TP I'm not blaming the 24v set up for damaging the batteries. You've not had these issues but others have. You need 24 v to start. Much less than that it it won't start. When one battery cell starts to go down, through age or premature failure, it affects the whole system and experience has shown that once one battery is struggling, it has a detrimental affect on the other. I can't say why exactly but it is so often the case. What I would say - again re charging not cranking, just look at the size of the cable that comes from the alternator. It's rubbish. Maybe only 16mm? I have swapped mine. So you can link the two batteries with a girder if you like, but the supply comes via a drinking straw. When cranking it makes no difference. But when charging, I believe that it does. Under normal use the batteries don't get depleted much, but we're talking about adding split chargers and all that. if you use the driver battery as a leisure battery then it's always going to be the unloved ginger-haired child compared to the other side. JMO

 

[Chas]

Digressing slightly,, I've just bought one of these chargers, https://www.screwfix.com/p/streetwi...0a-engine-starter-12v/3674r?_requestid=377229
My question is, you obviously can't disconnect the battery if you need to use the jump start function, but is that likely to damage any part of the system?

 

[Towpack]

You could use the jump start function on that charger as you would use a fully charged 3rd battery to jump from, just connect it across the driver's side battery as per the instructions in the owner's handbook.

 

[StarCruiser]

To add my to this I think many of the fail to start issues are to do with burned starter contacts, though the changeover relay does get a hard time and could fail. There's no rights and wrongs. Chris is correct with the simplicity of a 12v starter. The thing is, and it's Chris that pointed this out a while back, and was my bit of learning for the day, is that in properly charging a pair of batteries, or any number of batteries in parallel, as the 80 series ones are at all times other than starting, the - is connected to one battery at one end and the + is connected to the battery at the other end, thereby negating any losses in the connecting cables. The 80 actually does this by connecting the alternator + to the passenger side battery, while the negative is taken to the engine block from the driver's side battery. Any losses in the alternator connecting cable are reported back to and compensated for in the regulator in the alternator as there is a voltage sensing connection to the batteries via a fusible link.

There's certain advantages to a single battery setup and few disadvantages, especially when a leisure battery is on board to fall back on if the one starter battery goes down. Something that is potentially of little use if you've managed to kill both batteries in a dual battery system. Parallel batteries are notorious for dragging good batteries down if one goes bad and matched pairs are a must.

I'm going to point another thread to your well explained post on series charging Dave as it sheds light on some problems encountered on some of the 24v cruisers.

 

[Chris]

I'd never really looked at the Neg cables Rich. So it's diagonally wired, eh. I see. I thought my batteries both had earths. I added another earth by the way directly from the alternator and beefed up the one from the block as well as the cable between my cranking battery and the split charge over to the driver's side plus ... I know I know, a much bigger cable from the 120 amp alternator to the cranking battery.

 

[StarCruiser]

If that was the case, with both batteries having earths Chris, you would notice it the moment the main battery changeover relay changed over.

As the passenger battery Earth would be connected to the + of the driver side battery. It in fact earths through the changeover relay during non starting times.

 

[Bmonck]

These are great discussions for those of us on a steep electrics learning curve. Thanks guys.