Club 80-90 Forums

CJH wrote:Thanks Herb, that's quite enlightening. Just one clarification for the moment, while I absorb the implications: the 'voltage factor' you are referring to is simply the difference between the charging voltage and the battery's voltage? That's how I've read it, and your explanation makes sense if that's the case, but I just want to be sure.
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You got it.

CJH wrote:
Actually - a second clarification if I may. The 'automatic' drop off in current in a constant-voltage-variable-current charger - is this a natural consequence of the voltage difference reducing, or is it actively controlled by the charger? So if I have a dumb charger (such as I think the ZIG charger might be) which simply stays at around 14V, will the current drop off naturally as the battery voltage rises? This would seem even safer than my lower current 'intelligent' trickle charger (when used to charge two batteries in parallel), since it's not going to try and charge at higher, potentially dangerous, voltages.

A bit more to it than that and why we have the maximum voltage below the gassing point. but essentially yes caused by the voltage factor reducing below 1.


Fitting a changeover switch is easy all you have to do is use use a double pole change over switch, one pole selects between starter and auxiliary battery the other pole switches the earth off to the smart relay when the starter battery is selected stopping the smart relay energising.

Well...to do it right? battery to battery charger [very expensive!] two alternators? two diodes,one in each feed to battery [voltage drop!!].

I'll clarify the last big because I think I know what your thinking.
Although it's constant voltage charging. In practice it's slightly variable, When a charger or alternator is working at high current outputs the voltage will be lower due to resistance and temperature rise in the unit, It can be working at 1V less than the regulated output if you are checking for maximum regulated output this can only be done with a known good fully charged battery as 0.2V can make a big difference.

dave friday wrote:Well...to do it right? battery to battery charger [very expensive!] two alternators? two diodes,one in each feed to battery [voltage drop!!].

Yeah, I don't understand why the battery to battery chargers are so expensive. Prohibitively so if you ask me. Niche market I suppose.

bigherb wrote:I'll clarify the last big because I think I know what your thinking.
Although it's constant voltage charging. In practice it's slightly variable, When a charger or alternator is working at high current outputs the voltage will be lower due to resistance and temperature rise in the unit, It can be working at 1V less than the regulated output if you are checking for maximum regulated output this can only be done with a known good fully charged battery as 0.2V can make a big difference.

I had been considering using the ZIG's charger to charge both batteries in parallel, yes, but I've changed my mind. Since it's connected to the leisure battery it needs a charger on the starter battery to get the relay to close, plus the ZIG manual specifically warns against leaving it connected for long periods unattended. From what I've learned today though I doubt it would be a problem, but I think there's a simpler solution.

I think the LIDL trickle charger will do the job. It's lower current, so slower, and it's 'intelligence' will probably get confused by having the two batteries connected up, but by connecting it to the starter battery, courtesy of the smart relay I get a starter-priority dual battery top-up charger, with voltage and current specs that are very unlikely to cause me a problem. I found the following chart of the charging profile of a very similar charger (one currently available at LIDL*), and it seems like the charging voltage won't go above 14.4V, so I'm unlikely to have gassing issues (ok, *I* might, but the battery won't) even if the starter battery is full and the leisure battery is empty (which it won't be, because of my LVD).



I can probably take the same approach with a solar charger, provided I pay attention to its voltage profile - the typical current will probably be similar to the LIDL trickle charger at best.

*E D I T: No longer available it seems.

Mainly because they are a complicated piece of kit usually using an algorithm to keep things at the right level and often use a pulsed charge to charge the battery quicker. But for our application an adequately wired split charge system will work well at the fraction of the cost.

CJH wrote:Yeah, I don't understand why the battery to battery chargers are so expensive. Prohibitively so if you ask me. Niche market I suppose.

In addition to what Herb said, to get a a charging voltage of ~14 volts from a 12 volt battery you have to raise the voltage from the supplying battery. This is done in the same way as is used in a more common 240v ac (mains) inverter, using a voltage amplifying circuit called a "switch mode power supply". These are quite complicated and therefore expensive to produce.

dave friday wrote:Well...to do it right? battery to battery charger [very expensive!]

bigherb wrote:Mainly because they are a complicated piece of kit usually using an algorithm to keep things at the right level and often use a pulsed charge to charge the battery quicker. But for our application an adequately wired split charge system will work well at the fraction of the cost.

ghost123uk wrote: In addition to what Herb said, to get a a charging voltage of ~14 volts from a 12 volt battery you have to raise the voltage from the supplying battery. This is done in the same way as is used in a more common 240v ac (mains) inverter, using a voltage amplifying circuit called a "switch mode power supply". These are quite complicated and therefore expensive to produce.

I'm starting to come round to this idea after all. I've not looked at very many yet, but for instance, a CTEK D250S Dual + a CTEK SMARTPASS will provide all the functions of a split charge system, LVD, and MPPT solar regulator, and will intelligently manage the charging current to both batteries from the alternator, a solar panel and the ZIG charger.

It looks like I could just leave the ZIG charger on all the time when the van isn't in use, and wouldn't have to worry about one battery or the other over charging.

What other systems are out there for providing the same functions? I've heard of Sterling Power, so will look at their products. Anyone know which model(s) do all of the above?

I went for the CTEK units in the end, but I also added a CTEK intelligent charger to bypass the Zig's 'dumb' charger. Today I finished installing everything and rewiring all my consumers. It was a big rewiring task, and it's taken a few evenings, but I'm pleased with the result.

Here are the three units, all installed below the back of the Zig on the wardrobe wall. Together they provide mains charging of both batteries, intelligent battery-to-battery charging from the alternator, an 80A LVD for all the consumers, and an MPPT controller for a solar panel (yet to be added). Both batteries will be properly maintained from any or all of the charging sources, so hopefully it's a fit-and-forget-system.


I integrated the new units with the Zig as much as possible, but it meant some rewiring inside the unit. I reused terminals 1 and 2 (which used to take the input from the starter battery) to take the switched mains, so that I could wire the CTEK mains charger into them. The mains switch on the front of the Zig now does the same job that it used to, except that instead of controlling the built-in charger it now controls the CTEK intelligent charger. The mains fuse on the front panel also protects the CTEK charger. I reused the now redundant 'Touring/On-Site switch' as a Fridge power switch, and that meant feeding the switch terminals to a new chocolate block. And I took a feed from the CTEK LVD to terminal 4, so that the consumer circuits in the Zig can't flatten the leisure battery.


I got busy with my label printer, to help me remember what everything now does!


With a decent sized LVD I was able to wire in my inverter and potentially make use of the full 300W. I've installed it above the back of the Zig, and modified my mains wiring so that I can run the van's interior mains sockets either from the hook-up or from the inverter.


Apart from it all being completely automatic, fit-and-forget, and safe to leave hooked-up permanently, one of the best bits is the way it's tidied up my battery boxes.

Leisure battery:


Starter battery:


Next job is to fit a battery change-over switch so that I can add a spare leisure battery. It's now a lot easier to do this than it was before, since it'll just need a single chunky wire and a new earth strap. The switch I've bought has a '1+2' setting, and although the CTEK system might be OK maintaining both batteries in parallel I plan to simply use either battery 1 or battery 2 - by switching between them occasionally I should always have a reserve, plus putting the spare in the space next to the engine might help to correct my van's lean a bit!


I bought all my cable from a local place that was recommended to me on this forum - Vehicle Wiring Products Ltd. I'd never come across 'thinwall' cable before, and I'm still surprised how much smaller the outside diameter is for the same current capacity. For instance, that thin red wire feeding the auxiliary fuse box in my first photo above is 4mm2, rated at 39A

This weekend I mounted my spare battery in the engine bay compartment, and wired it up to the changeover switch in the wardrobe. I also installed a 100W solar panel on the roof above the wardrobe. I charged each battery independently till the end of the mains charger's cycle, so that they had both reached 'float' mode, and now I've turned the changeover switch to '1+2', so that they are joined in parallel. I've disconnected the mains charger, and I'm going to see how well the solar panel keeps them topped up in parallel during the week.

I'll keep an eye on them, but I think starting them off at the same state of charge (which also avoids high current between them when they're initially linked together), and only charging them with a few amps, should be safe. Obviously, drawing from them in parallel is quite safe, and it may well be that they will deplete at similar rates - in fact they'll tend to equalise anyway, since they're joined in parallel. But for bulk mains charging at high currents I'll probably still do them individually.

Not sure about charging them in parallel from the alternator. I want to get a feel for how much current flows into them in practice from my little 40A alternator, and I guess that the relatively short periods of a few hours will be quite safe.