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PMS3 Wiring Query


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Hi all, i'm new to this forum but have seen loads of good info about campervan wiring using the PMS3 power management unit.


I've been researching van electrics and have been working on the wiring plan (attached), but just have a couple of questions which i wonder if anyone might be good enough to help with?


Firstly, i have an eberspacher diesel heater which i've read should be wired straight to the battery as it should always be powered to allow it to shut down properly after use and burn off any combustion remnants (ie unable to be switched off anywhere other than the controller). Can anyone verify this is correct?


The 12v ports i'm using on the PMS3 are Pump, Lights and 2 x Aux ports (fridge is a Thetford compressor fridge which i'm also wiring direct to the battery, based on quite a bit of advice i've received).


For the pump, i want to link an extractor fan to come on at the same time. I've read the fuse sizes should be 7A for the pump and 3A for the fan. I've shown individual fuses for each of these, but the PMS3 has an inbuilt fuse for the pump output... Would it be right to put a 10A fuse in there to cover both 7A and 3A for pump and fan?


Similarly, there are 2 Aux outputs and i will be using one for the fan and one for the cooker ignition, both of which need 5A fuses. But, the unit only has a single built-in fuse for Aux... I assume this is for both outputs combined, so as i have 2 x 5A fuses, is it right to put a 10A fuse in here, same as above?


Any feedback or comments that anyone has on the rest of the set-up would be welcome and greatly appreciated.


Thanks in advance




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Samwise-zambeezi - 2021-03-24 3:03 PM


Firstly, i have an eberspacher diesel heater which i've read should be wired straight to the battery as it should always be powered to allow it to shut down properly after use and burn off any combustion remnants (ie unable to be switched off anywhere other than the controller). Can anyone verify this is correct?

Hi Sam and welcome,


I can't comment on your fusing questions but can confirm that your Eberspacher heater absolutely MUST be wired with a permanent 12 volt live connection to allow the heater to complete its cool down cycle as you stated.


I'm sure others will be along soon to answer questions on your fuses.



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The PMS3 unit is now marketed through Bonus Plug-in Systems






but I’m guessing the current version differs little from the original for which there is an Installation and User Instructions document here




This link may also be useful




I assume the amperage of the fuses that protect the pump, lights and auxiliary 12V ports are shown on the PMS3’s front. This is an ‘old tech/caravan’ piece of kit, so care will be needed when uprating fuses beyond their original specification.


There has been some earlier forum discussion about the PSM3



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Thanks for your responses guys.


I'd read most of what i can find on various forums about the PMS3 but not quite found the answer to the fuse question i have.


The thing is, i'm not actually uprating any fuses beyond the capacities shown on the the unit, i would be fitting those exact fuse sizes as recommended, but then adding smaller fuse sizes down the line as the feed splits to power the various multiple bits (ie. pump 7A + fan 3A = 10A pump fuse size in the unit, and 5A fan + 5A cooker ignition = 10A aux fuse size in the unit).


As long as the wire i use is of a high enough capacity (ie. 10A), then i'm assuming/hoping the smaller fuses will protect the bits of equipment with reccommended fuse sizes smaller than those built into the PMS3 *-)

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Every wire/cable that is connected to the battery positive should/must be protected by a fuse of suitable rating to protect the wire/cable. The fuse/fuses to be positioned as near as practical to the battery terminal. Not doings so increases the risk of electrical fires in the vehicle. There are cables to the PMS3 from the batteries that are not fused, as is the feed to the solar controller.





To avoid multiple connections at the battery positive post a 'master' high rated fuse feeding a HD cable to a distribution fuse holder is recommended. Using buss bars for multiple connections, for example the multiple connections to the chassis and leisure battery negative.




As you are discovering the PMS3 is not best suited for a modern system, its a dated 1980 design for a very basic electrical system in a caravan. It would be easier and safer to have a separate fuse box for the feeds to appliances.


The PMS3 has a poor quality battery charger, at 13.8 volts it really only provides a float voltage and DC supply when on hookup. The design was never intended for large battery banks. However the solar controller will provide a good charge.


I note you have an alternator charge control via a relay switched by D+. On some vehicles D+ is not easy to find and using a voltage controlled relay would be an alternative.


You need to ensure the cables used have low volt drops for units like the heater and the compressor fridge, I suggest 6mm2 cable for these. Cable 16mm2 for the battery cables carrying high current from the engine battery via the relay to the leisure battery, the negative link between the leisure batteries and chassis connection. Fuses in this high current path to be 50 to 80 amps , link fuses.



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  • 2 weeks later...

Hi Mike,


Thanks very much for such a helpful post, really appreciate you taking the time. Your advice is well received and the points you mention are all duly noted; i've got a split charge system made up using the heavy duty 16mm cable, and intend on ordering all the rest of the bits i need (including 6mm cable) from 12volt planet once i've figured out the minor details.


I've been looking into the missing fuses in my plan and have hit a few stumbling blocks...


Firstly, could you please advise whether i need a fuse between the two linked leisure batteries? Most of the stuff i've seen on the internet say you don't, but there's the odd post that says you should - which is enough to throw a spanner in the works for me. The batteries are positioned immediately next to each other, if that makes any difference?


Regarding the batteries to PMS3, i've consulted the PMS3 instructions i downloaded and it shows the fuses you correctly mentioned. The sizes are 15A from car battery and 25A from the leisure batteries. Not sure why but i might have expected bigger fuses?


Regarding the solar fusing, i'm reading that i need one both between panel and controller (near to panel), and also between controller and battery, near to the battery. However, Just to confuse me even more, when i checked the instructions for the solar controller - it says put the fuse between controller and battery on the negative line, not the positive! See extract below:


"Note: although the solar controller has built-in electronic protection,

for safety and added protection please install inline fuses into the

circuits between each battery and the controller, on the negative

wire, as close to the batteries as possible. Current rating of fuses should be chosen according to the maximum power current / short

circuit current of your solar panel."


Does this ring true with anyone else? Lastly, any idea what sort of fuse sizes would be appropriate for a 250w Solar panel?


I've updated my plan to show the fuses mentioned with the sizes based on what i've read on the web. I've also added a second run from the solar controller to my van battery, as i found i can set priority charging to, say, 90% leisure battery, 10% vehicle battery.


Thanks in advance




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“could you please advise whether i need a fuse between the two linked leisure batteries?”


Where the batteries are next too each other using well protected cables, the risk of a short circuit is low and many installations are wired with only one 'master fuse' from the battery pair. Ideally a separate fuse on each battery positive would be safer and a space saving method is to use the 'cube' fuse direct on the battery post.




“ are 15A from car battery and 25A from the leisure batteries. Not sure why but I might have expected bigger fuses? “


These fuses are correct for the currents taken by the PMS3.


“Regarding the solar fusing, i'm reading that i need one both between panel and controller (near to panel), and also between controller and battery, near to the battery “


Any fuse between the controller and a single panel would require very unusual circumstances to 'fuse', however its a useful addition if only used to isolate the panel whilst working on the system.

The fuse value should be higher than the rated short circuit current of the panel. A typical panel of 250 watts would have a SC current of around 10 amps so a 15 amp fuse would be suitable. This fuse should be fitted near the controller not panel.


The fuse between the controller and battery should be at the battery end of the cable. This should be rated slightly higher than the expected current from the controller. With a 250 watt panel this would vary depending on the type of controller. With a quality controller this could be in the order of 20 amps.


“install inline fuses into the circuits between each battery and the controller, on the negative

wire, as close to the batteries as possible.”


This is an unusual set-up for fusing for a dual output controller. This suggests the controller may not be suitable for your system and panel. If you provide details of the controller and solar panel I can advise.



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The panel is 200 watts and at maximum power output is specified at 18v, 11A, with a maximum current SC of 11.9A. So the recommendation of a 15A fuse in the panel supply lead is valid.


The controller you have is not the best technology being a PWM type, this type limits the current to the battery/s to a maximum of that provided by the panel, in this case, 11 amps. Thus the maximum power into the battery will be approximately 140 watts ( 13V x 11A). An alternative controller type, a MPPT controller, will take in the maximum power from the panel and convert to a voltage for the battery. Using the same battery voltage of 13v, a MPPT controller would deliver a maximum current of 15 amps (200 W / 13 V). (In practice there will be cable, conversion losses, panel losses with temperature, panel alignment loss, and solar available, so the maximum will be rarely seen).


Under typical solar conditions a MPPT controller will supply around 30% more power to the battery and camper loads than a PWM type.


If its possible I would change your solar controller to one of the units recommended below. Not only will they extract maximum power from your panel, they have considerably better control of charging parameters, ensure a correct charge and will not cause damage to your battery.


For a dual battery solar controller the Votronic is recommended,




The best performing solar controller is (single battery) the Victron Smart 75/15, ( set-up and charge info via a phone app)




With a single output controller to achieve a maintaining charge to the engine battery, a battery master would be needed,








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Sorry mate, me again!


So, i'm looking at this fuse box:




From it i would connect my diesel heater, compressor fridge, PMS3 unit, and...


My question is, am i ok to connect the solar controller to this fuse box as well (with 15A fuse)? Don't know why but i'm wondering whether having load coming the other way, from the panels to charge the battery, would make any difference to the set-up? May be a daft question.


Provided that's ok, i'll then have from my main leisure battery, a 16mm2 cable with in-line fuse from split charge relay, a 16mm2 cable linking to secondary leisure battery and then a 16mm2 cable with master fuse to that fuse box. So i'm thinking i'll need a triple battery terminal like so:




In fact, my secondary leisure battery will need one as well, won't it? As it will have a 16mm2 cable linking to main leisure battery, the big cable from the solar controller, plus a 16mm2 cable to link to the fuse box negative bus bar.


Also, do you happen to know what size cable i would need to link car and leisure batteries to the PMS3 (15A and 25A fuses respectively, as mentioned before)??


Thanks again for all your effort



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Connecting the solar controller to the same fuse box that is feeding other circuits is OK, there is no problem having 'in' and 'out' via the same fuse box.


Multiple studs on a battery terminal may be needed but its possible the get 2 ( perhaps 3) cable terminations on one stud. However having multiple studs can make a neater installation.


I don’t understand the comment “big cable from the solar controller”. The connection to the battery should be via a fuse. In your system 4mm2 or 6mm2 cable from controller to battery and system negative.


For the connections from the batteries to the PMS3 use 4mm2 cable.


What vehicle is the base for your conversion?

(some models are 'fussy' as to the process of connections to, and charging of, the engine battery).



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The 'big' cable i was referring to is the 4mm2 solar cable, with a greater diameter due to double layer insulation. I was actually talking there about the connections from my leisure battery 2 negative terminal; my solar controller positive will connect to leisure battery 1 positive (via fusebox), but the solar controller negative will go to leisure battery 2 negative (as batteries linked in parallel).


So my leisure battery 2 negative connections will be: leisure battery 1, solar controller, and van chassis earth... which is why i was wondering about the triple terminal there.


Thanks a lot for the advice Mike, you've been a great help.


The van is a mk6 transit, btw

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