Dual Battery System with Solar Cell - Instructions

Let's deal with something completly new: the car. Better, power in the car. Currently I am converting my Land Rover Defender 110 into a camper van. Since I want to be independent of shore power, I have been looking for a solution for a self-sufficient battery management and found one. I would like to introduce it here.

Doppelbatteriesystem Hardware

Definition of a dual battery system (DBS)

What is a DBS and why do you need it anyway? With a DBS 2 batteries are installed. A starter battery and a utility battery (or auxiliary battery). The goal is to have more capacity and power available.

Difference between starter and utility battery

The starter battery is the battery that starts the engine and is always charged via the alternator (LiMa).

On the other hand, the utility battery provides power for additional consumers (e.g., heater, refrigerator, lights, electrical outlets, etc.).

Battery parallel / series switching

There are two different approaches in building such a DBS: parallel connection or disconnection via a relay. With the parallel connection you need 2 exactly the same batteries (same manufacturer, same capacity (Ah), best same series and batch). Then the two plus and minus poles are connected to each other and so you get twice the capacity. Advantages of parallel connection:

  • very simple
  • no additional hardware required
  • bigger capacity

The disadvantages:

  • if onebattery breaks down, both must be replaced
  • if deep unloading, car can not be started anymore

Disconnection via a relay / isolator

When disconnected via a relay, the two batteries are connected together, e.g. the engine is running and enough power is flowing to charge both batteries. When the engine is off, the relay disconnects the two batteries so that the starter battery always has enough power to start. The advantages are:

  • Flexible usage
  • Various batteries usable (manufacturer, capacity, type)
  • Low load & protection of the starter battery

The disadvantages:

  • more expensive
  • extra hardware necessary (relay, possibly monitor)
  • possibly not as much capacity available as with parallel connection

Since the separation via a relay in my eyes is the better solution, I have decided to do so.

T-Max DBS02 vs IBS DBS vs CTek D250SA

There are many different solutions to realize such a separation via a relay. Very easy using a normal relay (for example, Victron Cyrix-ct) and a battery monitor (for example, National Luna). However, I miss the extra features there, like a manual link between the batteries. So I took a closer look at the following finished solutions. Here is a comparison of the 3 most popular:

CTek D250SA

By far the simplest finished solution delivers the CTek D250SA. There are only 4 connections: + Pol starting battery, + Pol supply battery, grounding (= -Pol of the starter battery), + Pol solar cell. Simply connect the 4 cables, done. It is not even a charge controller for the solar cell necessary. Everything is done by this little box.

Problem: Expensive (~ 300 €) and it can only with max. Load 20A. However, the LiMa delivers over 100A. The relay in the CTek throttles the charging speed of the 2nd battery down to 1 / 5th - meaning it takes 5 times to charge as connected directly to the LiMa.


The IBS DBS is a reliable and well-engineered DBS. It comes with monitor and high performance relay with 200A. You have a lot of wiring (4x monitor, 3x batteries), but the effort is worth it. It makes a "beep" in case of errors or deep discharge, you can manually link the batteries, etc. The only drawback, the price of 350 €. If money does not matter, buy it!

T-Max DBS02

A cheaper alternative is the T-Max DBS02. It offers almost the same features as the IBS DBS - can even up to 250A. In terms of processing, the IBS DBS is in no way inferior. Price: 130 €. 220€ cheaper than the IBS DBS. Although there is no acoustic signal or the convenient deactivation of the auto-link function by pressing a button. Still unbeatable for the price.

Shopping List: All you need for the DBS with a solar cell

In addition to the relay itself, of course you need a few more components. Solar cell, cables, connectors, fuses, etc. I list everything here, what I have used:

Of course you can replace all components with similar ones. In particular, the AGM batteries (in my VARTA, like Running Bull) you have to choose for space and power requirements. I bought the solar cells from OffgridTec. Good service and advice!

The best battery type: AGM vs Gel vs LiFePo4

Finding the right battery depends entirely on your project and budget. AGM and gel are similar, with AGM being slightly cheaper. AGM are also suitable as starter batteries, gel not necessarily. Both batteries should not be completely discharged. AGM has a slightly higher charging voltage (14.6-14.8V) than gel batteries (14.4V).

The biggest advantage of LiFePo4 batteries is the possibility to completely discharge them. A 100Ah battery also delivers net 100Ah and not 50Ah like a 100Ah AGM battery. These are unfortunately not suitable as a starter battery and thus useless for this battery system.

Eine 50Ah-LiFePo4-Batterie kostet ca. 800€! D.h. ich kann vier 100Ah AGM Batterien verschleißen bis sich ein LiFePO4-Akku gelohnt hätte. Da ich von einer Lebenserwartung von 5 Jahren bei guter Pflege (regelmäßig über ein CTek-Ladegerät mit IUoU-Kennlinie komplett laden) gehen 20 Jahre ins Land... und solange glaube ich nicht, dass eine LiFePO4 lebt. Dazu kommen ein anderes Ladeschma und ein anderes Batteriesystem, und und und... 

A 50Ah LiFePo4 battery costs about 800 €! That I can wear four 100Ah AGM batteries until a LiFePO4 battery would have been worthwhile. Since I expect a life expectancy of 5 years with good care (regularly charge using a CTek charger with IUoU characteristic) it will take 20 years to break even ... and I do not believe that a LiFePO4 works that long.

Calculate power consumption

Before you deal with the installation, you should be aware of how much electricity your consumers need at all. Based on this number, you can determine the size of the supply batteries.

Most devices indicate the consumption in watts (= current), whereas the capacity of the batteries is given in Ah (= ampere hour). Therefore, you have to convert:

Watt = Current * Volts
Current = Watt / Volt

Strom sind die Ampere und Volt i.d.R. 12V (oder 24V). Du rechnest Dir also aus allen Verbrauchern die Strom-Zahl und multiplizierst sie mit der Anzahl der Stunden, die die Batterie pro Tag laufen soll:

Current is the amperes and volts is 12V (or 24V). So you calculate the current from all consumers and multiply it by the number of hours the battery should run per day:

Consumer Watt Volt   Ampere Duration / Day Ah
Fridge 42W 12V 42W / 12V = 3,5A 8h 28Ah
Lights 20W 12V 20W / 12V = 1,66A 2h 3,33Ah

I have created an Excel spreadsheet, which calculates the consumption. You can download it for free: Consumption Calculation Excel

The battery should not be completely discharged, so take the result of Ah with 1.5 (= 50% more consumption than necessary). In my example above: 28Ah + 3.33Ah = 31.3Ah * 1.5 = 46.95Ah. Thus, a 100Ah battery can run well at least for 2 days.

Wiring double battery system

So much for the theory. Well, almost at least. Below you find a circuit diagram, how you should wire everything (in relation to the DBS of T-Max - otherwise there may be other colors / connections).

Circuit digram DBS with solar cell

The wiring diagram looks complicated at first glance, but it's not too hard. Everything below the dashed line you can ignore at first - please click on it for a large view:

Schalplan DBS

Connection relay

I'll go through everything step by step and explain it as well as I can. On the relay there are 4 screws to attach the cables (2 times thick, 2 times thin). Each is numbered 1-4.

1) The supplied very short cable is connected to screw 1 and 3. With this cable, the relay is switched automatically with sufficient voltage.

2) The green cable of the DBS02 (= control unit of the battery system) is screwed right here. The 2 amp fuse is included.

3) In addition to the cable of 1) comes here the cable of the plus pole of the starter battery. Shorten the supplied 15qmm cable accordingly and crimp the eyelets.

4) Connect the cable from the positive pole of the supply battery here. As with 3) shorten first and crimp.

Verkabelung Relais T-Max

All in all, only 4 cables need to be attached to the relay. Thanks to the numbering this is not a problem.

Wiring DBS02

The control unit "DBS02" is like the brain of battery control. In addition to the supplied charging current, the charging states of both batteries are displayed and whether they are currently gelinked (= interconnected).

The control unit has 4 cables already connected at one end to a plug. Since the 1.5m are too long for me, I braided the individual strands:

  • Black cable: Connect to the ground (=minus pole of the starter battery)
  • Red cable: Connect to the positive pole of the starter battery
  • Blue cable: Connect to the positive pole of the supply battery
  • Green cable: Connect to port 2 of the relay

The controller measures the voltage of the starter battery or supply battery via the red or blue cable. In addition, it receives the power to its own operation from the red and black cables. It switches the relay on / off via the green cable.

Conclusion DBS02

As you can see, the dual battery system DBS02 from T-Max is easy to use. That can be installed by everyone! All required parts are included. Only the tool (crimping tool, etc.) you have to buy / borrow / you have already there.

Note: Grounding means the negative terminal of the starter battery. Everything can be connected there. So also connect the minus of the supply battery with the minus of the starter battery. The same applies to the charge controller of the solar cell and to all consumers.

Wiring solar cell

I used a MPPT charge controller with built-in Bluetooth. An MPPT charge controller is slightly more expensive, has better charging efficiency, and can convert the 36V to a 12V system, which can bring out more power in darker light. Thanks to built-in Bluetooth no extra display is necessary - this saves costs!

The above linked solar cell and the 7m cable comes ready with power connections via a MC4 connector - there is just one way to wire them up. Put the other end in the charge controller:

Anschluss SmartSolar Laderegler

BATT: means the battery. So here are the two connections for the battery cable. The other two ends are clamped on the plus or minus pole of the supply battery. Again, there is already a fuse in the cable.

PV: in the red box stands for photovoltaic. Connect plus and minus of the solar cell. First the battery should be connected to the charge controller!

LOAD: you can connect another consumer up to 20A. But I do not use it.

SmartSolar settings for AGM batteries

In order to charge the battery with the correct settings, you have to set up the SmartSolar controller. You connect via Bluetooth with the charge controller (PIN: 000000). Then you download the app (just search for Smartsolar in AppStore / PlayStore). After a few firmware updates, you finally get into the menu. There you can see the current state of the solar system. Click on the cog in the upper right corner and then on "battery". There you can set the charging voltage. For an AGM battery, 14.6-14.8V is recommended.

When does the relay link / unlink?

You have now successfully installed the dual battery system and the solar module. If you now start the engine, the relay should switch after a few seconds (it makes a "click"-noise once) and on the control unit "Linked" lights up. This means that both the starting and the supply battery are now charged via the alternator.

If the engine is off, but the sun is shining, the utility battery will be charged via solar. If this battery is fully charged or the power is strong enough, the relay also links the batteries and the start battery is charged via the solar cell, too.

It is therefore charged in both directions as soon as the voltage of LiMa or solar system > 13.3V.

Connect battery protection and consumers

Now you almost made it! From now on, the circuit diagram is about the lower part. Next you need only one cable from the positive pole (I have used the remaining 16mm cable from the T-Max) to the 100A fuse and from the fuse to the input of the battery protection. From the output, I once again put a cable to a 100-A fuse. Finally it is connected to the fuse box.

I am not sure if the 2nd 100A fuse is necessary, but for sure it is safe. At the fuse box all positive poles of consumers are connected. The negatives of the consumers can be clamped together to form a line and can be connected to the negative of the starter battery.

Tips and extensions

Between the supply battery and the first 100amp fuse I installed a switch. So I can easily disconnect all consumers from the mains (useful, if the car is not used for a long time).

For the individual consumers, I have also installed some rocker switches. This allows me to switch individual sockets (such as those that go outside) or turn the interior lighting on / off.

When disconnecting a battery always disconnect the negative terminal first, then the positive pole.

When connecting a battery first connect the positive pole, then the negative pole.


Even if it looks very complicated, it is not that bad. The important thing is that you do not mix up plus and minus when plugging together, otherwise there is a short circuit and the car electronics can - in the worst case - say goodbye completely.

In my opinion the whole thing is practicable by everyone and you do not need to charge an expensive workshop. Just do it! ;)

A few more pictures of the finished system:

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- Bosch S5 A13 + Optima


Hallo Johannes,

vielen Dank für die super Erklärung :). Ich bin leider Laie, evtl. kannst du mir noch 2 Fragen beantworten?

Würde die Anlage auch mit Kombi Bosch S5 A13 AGM 95Ah als Starterbatterie (Bereits eingebaut) und Optima rot rtc4,2 50 Ah AGM 50 als Verbraucher funktionieren (kleiner, daher mehr Platz unter Sitz), oder muss ich da noch etwas zusätzlich beachten?

Ich hätte gerne ein flexibles Solarpanel, dass ich fest auch Hartschalen Dachzelt montieren will. Hast du dazu auch eine Empfehlung? Ich habe z.B. folgende Kombi gefunden: https://www.bau-tech.shop/wohnmobilanlagen/wohnmobil-anlagen-100-130watt/100watt-wohnmobil-solar-set-flexibles-solarmodul-victron-laderegler-bluotooth.html. Müsste doch gehen oder?

Vielen Dank schon mal

Gruss Oliver

Antwort von Johannes - 05.07.2020

Hi Oliver,
solange beide Batterien 12 Volt haben und beides AGM Batterien sind, sollte das m.M.n. gehen. Übernehme aber keine Garantie ;)

Das flexible Panel geht in jedem Falle auch. Das Kit von Dir hat auch alle Kabel schon dabei, daher ist das sogar noch einfacher, da du mit den Kabelstärken nichts falsch machen kannst :)

Viel Erfolg!
Schöne Grüße

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- Frage zum T-Max


Hallo Johannes, zunächst mal vielen Dank für Deinen tollen Bericht, der mir wirklich sehr weitergeholfen hat.
Meine Frage: Wo und wie hast Du das Relais befestigt? Die offenen Kontakte machen mir da etwas Sorgen. Auf den Fotos kann man dazu leider wenig erkennen. Wo hast Du das Display befestigt?
Schlne Grüsse

Antwort von Johannes - 08.06.2020

Hi Gary,
freut mich, dass dir der Bericht geholfen hat!
Das Relais sitzt im Batteriekasten unterm Fahrersitz. Dort kommt ja nichts dran. Die Anschlüssen der Batterien ist ja auch ganz frei. Sehe da kein großes Problem.
Das Display habe ich nicht verschraubt, sondern es liegt einfach rechts neben dem Fahrersitz und der Cubby-Box. Das reicht mir.
Schöne Grüße

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- Ladung der Starterbatterie durch Panel verhindern


vielen Dank für Deinen ausführlichen Beitrag. Ich habe das System auch eingebaut und mit einem Solarpanel kombiniert. Das nachladen der Starterbatterie durch das Panel, wenn die Versorgungsbatterie voll ist finde ich allerdings nervig und unnötig. Ständig schaltet das Relai. Hast Du eine Ideen, wie man das verhindrn kann?

Danke und Grüße, Tim

Antwort von Johannes - 08.06.2020

Hallo Tim, das ist weiß ich leider nicht und spontan fällt mir auch keine Lösung dazu ein.
Wenn dann könntest du einen einen Schalter einbauen, um das Relais komplett zu deaktivieren...dann musst du aber immer dran denken, es auch wieder einzuschalten.
Schöne Grüße

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- Solarzelle parallel oder Reihe


Hi Johannes,
wie würdest du 2 Solarmodule verkabeln, Reihe oder parallel, beim MPPT 75/15?


Antwort von Johannes - 05.05.2020

Hi Holger,
gute Frage. Spontan würde ich sie in Reihe schalten. Die Solarzellen liegen beim Auto so dicht beieinander, dass wohl meistens eh beide in der Sonne oder im Schatten sind. Die vielen Kabel bei einer Parallelschaltung wären fürs Auto bei mir zu viel Kabel ;)
Schöne Grüße

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- Tiefentladeschutz


Erstmal danke für diesen Beitrag - da ist wirklich alles top beschrieben und man kann sich wirklich super daran halten!
Zwei Fragen hätt ich:
1) Kann man den Tiefentladeschutz einstellen (bzw. welche Einstellung würdest du empfehlen)?
2) Kannst du mittlerweile sagen, ob bei 100A-Sicherungen (also vor und nach dem Tiefentladeschutz) notwendig sind?
...in manchen anderen Schaltplänen ist nur die Plusleitung zum Entladeschutz abgesichert, die Leitung zum Sicherungskasten dann nicht mehr.

Danke schon mal,
LG Manuel

Antwort von Johannes - 05.05.2020

Hi Manuel,
danke für deinen Kommentar und entschuldige die späte Rückmeldung...
1) Ja, den Tiefentladeschutz kann man einstellen. Die richtige Einstellung ist abhängig von deiner Batterie und wie sehr du sie auslasten willst ;) Es liegt eine ganz gute Anleitung dabei in der das erklärt wird.
2) Ansich reicht nur eine. Der Vorteil der zweiten ist, dass falls mal mehr als 100Ampere durch die Leitung gehen, der Tiefentladeschutz heile bleibt und nicht überstrapziert wird.
Schöne Grüße

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- Ladespannung AGM / LiMa


Sehr gute Beschreibung des DBS. Eine Frage habe ich noch: die AGMs sollten mit 14,6 V geladen werden, wieviel liefert deine LiMa/Regler? Macht es nichts, wenn die Starterbatterie nicht voll geladen wird?

Grüße, Sascha

Antwort von Johannes - 31.10.2019

Hallo Sascha,
freut mich, dass dir die Beschreibung gefällt!
Die Lichtmaschine liefert nicht ganz die 14,6Volt - die schwankt immer irgendwo zwischen 13,5 und 14,4 Volt.
Der Solar-Laderegler und auch das Batterieladegerät haben eine spezielle Ladefunktion, mit der beide Batterien voll aufgeladen werden und die Haltbarkeit der Batterien verlängert werden.
Schöne Grüße

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