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Senior Member
Posts: 150
Reply with quote  #1 
This (candidate) modification applies to instances that have a BMS 1215 installed but the same problem is very likely also the case for earlier produced campers. If you are not impressed with your battery performance and find the batteries empty sooner than expected, then the below text may provide some answers.

If you read the BMS 1215 user manual it explains in section 2.5 on page 21 how the BMS needs to be installed when using two batteries in parallel and the special way in which those two batteries need to be connected to each other. Many other documents, amongst others some books written by Collyn Rivers (Solar that really works), make the same recommendations.

In my 440 the batteries are connected using the wiring schema on the left, i.e. the WRONG way. The battery on the UEV driver side is connected to the BMS, and also has all users connected to it and the battery on the passenger side is connected 'in slave' configuration to the battery on the driver side.

You can easily check this for yourself if you take off the little battery panel on the driver side.


You'll see lots and lots of red cables connected to the plus of the battery and ...

you will see a single thick black wire connected to the minus pole of the battery (I suspect this is the cable that runs to the passenger side battery) and a second short tick black cable going from the minus pole to the shunt (part of the BMS system) to which then a lot of black cables are connected. The shunt is the component visible at the top of the picture. In the middle section of the shunt you'll see two thin cables (red and black) which are the sensing wires that measure the charge/discharge current that feed information to the BMS monitor system.


If you'd open the battery compartment on the other side you'll see a single thick red cable connected to the plus and a single thick black cable connected to the minus of the battery and both disappear upwards towards the cavity above the back door. I haven't checked this yet, but my guess is that these two wires are routed all the way to the other side and are indeed connected to the plus and minus of the driver side battery.

You can also see the temperature sensor on the right hand side in the picture, connected to the minus pole of the battery, and the thin red wire is the voltage sensing wire.

RedArc - in compliance with best industry practices - recommends to connect the BMS and electrical consumers according to the drawing on the right (marked as CORRECT).

Why is this important?

The trouble comes from the two wires (named a and b in the top right drawing) that connect the two batteries to each other. Every electrical wire has a resistance, and as soon as a current flows trough that wire it creates a voltage drop. The thinner the cable, and the longer the cable length, the higher the resistance and hence voltage drop.

These voltage drops are BAD NEWS in 12 Volt land. In fact a voltage drop of 0.3 Volt is already a big problem. This is because a batteries' effective working range is somewhere between 11.5 and 12.5 Volt, or max 1 volt or so. A 0.3 V voltage drop therefore is 30% of the working range and many devices, such as a fridge or pump, will work noticeably less efficient as the voltage gets lower, and as a result will be pulling more amps to achieve the same refrigeration/pumping effect. In other words, even though a battery may be fully charged, because of the voltage drop a fridge will see a battery that looks like it is already 30% discharged. Think of a flashlight that becomes weaker and weaker as the battery is drained.

When charging the two batteries a similar problem occurs. The battery that is closest to the BMS will get the full charging voltage and will be charged fully, but the other battery will see a lower voltage and hence won't be fully charged.

Effectively we have a setup that - on paper - is 2 x 105Ah, but in reality we have one battery operating at 105Ah and the other one, due to all the charge and discharge losses, behaves like a 80Ah battery. If we take into consideration that we shouldn't let our battery capacity go below 70% (to maximize battery life) then a battery that has such a cabling loss is more weight than value.

In my UEV 440 this problem is very likely to occur because the two batteries are nicely separated to the left and right side of the camper. This is fantastic for weight distribution, but the two batteries are connected to each other by a cable that runs over the rear door. If we add up the length of cable for the plus and minus cable the total length of extra cable (a + b in the diagram above) is somewhere in the 6 meter of cable length. Fortunately, the cable is of a decent thickness (AWG 6 or 16 sq mm) but with a 15A charge current into the batteries or a 20A discharge current taken out of the batteries there is simply going to be a voltage drop.

To check my thesis in practice I fully charged by batteries, then switched off 220V and switched on all electrical consumers, i.e. all led lights, the fan, my 78L ARB fridge, the Webasto HWS and the waterpump. All together this consumed 20A (according to the BMS 1215). When doing the maths for an estimated 6 meter of cable @ 20 Amps current and 16 sq mm cable the voltage drop is calculated as: 6 x 20 x 0.0164 / 16 = 0.123 V. I then measured the voltage drop between the two minus poles and between the two plus poles using a multimeter. Both had a voltage drop of ~50mV, i.e. in total approximately 0.1 V, which is pretty much what I calculated as well.

So far I have ignored the voltage sensor, but this makes things even more interesting: the voltage and temperature sensor is connected to the passenger side battery (the one that is from electrical wiring perspective furthest away from the BMS and has the extra voltage drop). The BMS is using the voltage sensor to compensate for the voltage drop by increasing the charging voltage above what is recommended for the battery. For the passenger side battery this is fine, since the cable loss eats that extra voltage away so the passenger side battery gets exactly the maximum voltage it should get, but the driver side battery is 6 meter closer to the BMS and hence subject to an extra 0.12 volt charging voltage, so effectively is always charged at a too high voltage, reducing it's life.

So, my conclusion so far is that when charging the batteries both are fully charged but the battery on the driver side is overcharged which will reduce it's battery life, and when discharging the passenger side battery is disadvantaged by an additional 6 meter of wire, which will make it seem like it's an 90Ah battery instead of a 105Ah one (depending on the discharge current) and hence the batteries will run out of steam sooner than I'd prefer.

Why do I care about all of this? I hate generators and prefer to camp on unpowered sites, so I want to maximize the efficiency of my system. Even though I'll be installing an over-dimensioned solar system (I have 540W on order) I still want to make sure I maximize the whole setup so that I can camp for prolonged periods of time under various weather conditions. The 0.12 V voltage drop is 'reasonable' (i.e. I'm certainly not suggesting that Conqueror is doing a bad job), but it can be done better and the way the batteries are wired together is just wrong.

To fix these issues I intend to make three modifications:
  1. I'm going to connect both batteries to each other via the shortest route, which is under the floor in a direct line from one battery to the other. Obviously the + will be connected to the + and the - to the -. I'll use a dual 16 sq mm tinned copper which is the same as what Conqueror uses, but the length of cable will be a third of what is currently used and hence have less voltage drop. The length of cable needed is 1800mm. See:
  2. Furthermore, and most importantly, the shunt on the negative side is currently connected to the - of the battery on the driver side. I'll remove that connection and instead connect the shunt to the - of the battery on the passenger side. Again, under the floor but using an even thicker 25 sq mm tinned copper. The length of cable needed is 1600mm. See:
  3. I'll double up the connection between the BMS and the driver side battery by reusing the existing cable that currently connects the two batteries (i.e. the thick red and black wires that can be seen in the third picture above).
I intend to run the wires through a protected area under the floor, directly behind the door ...

In this picture you can see an existing cable (in the center of the picture) that is already running through this cavity. The existing cable is for feeding the power outlet on the outside of the 440 (behind the passenger side wheel).

Here you see the area just under the door that I'll use to connect the two batteries via the shortest route.
IMPORTANT: If you have no idea what I am talking about then this is a modification I most definitely do NOT recommend you to undertake yourself. Go to a qualified car electrician.

An 0.1 V voltage drop is also not earth shockingly bad. I can do this modification myself and it'll cost me $50 for new wires and a Saturday afternoon. If you need an electrician you need to balance the investment cost versus the gain (a system that works a little bit better). If the choice is between making this change, or adding an extra solar panel on the roof, then the latter is probably a better investment.

I'll update this thread with some more info and pictures in the near future.


Posts: 66
Reply with quote  #2 
Thanks 4Fun. Very informative.

I'm in the process of making a list of things to fix on my 440 so that I can get an auto electrician in to do the whole lot at once. This is definitely something I'll now be looking at.

Can you please give me a bit more detail on exactly what wire you'd suggest and the length needed to fix this issue. I live in a very remote location, so I'm going to have to order every part I need online and then get an auto sparky in to do the labour.

Many thanks.


Senior Member
Posts: 150
Reply with quote  #3 
Added actual measured data, cable types and lengths to the text.

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Posts: 50
Reply with quote  #4 
hi 4fun
your theory is ok if you use small wiring
but the size of cable and the short distance between the batteries wont make voltage drop a problem

I reckon you might be wasting your time rewiring

Lc200 + Uev 490

Senior Member
Posts: 150
Reply with quote  #5 
Hi Razor, yes that's what I concluded as well at the end of the long story. "0.1 V voltage drop isn't too bad". I still intend to fix this at some point, but that's because I can comfortably do it myself, and it's going to cost me approx $40 on new cable. If you need to involve a car sparkie to do the job for you, then don't bother.
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