Connecting Motorhome and Caravan Batteries
As is often the case, the inspiration for this article came from talking with a fellow motorhomer. He was under the impression that connecting batteries in parallel would shorten their life. He even had a very good explanation for this …
“As one battery discharges, it will drag the other battery down, then that second battery will drag the first one down. Before ya know it they have dragged each other to death” (not really word for word – but you get the idea”
To avoid this issue, he was discharging one battery, then switching over to the other – charging them separately then going through the entire process again. There are a number of things wrong with this approach … but let’s just focus on the reason he was doing this.
This concept of one battery discharging the other was unknown to me so I started doing some research to see if their behaviour is real or just another motorhome/caravan myth.
Before I start – let me first explain the two connection methods when more than one battery is used in a caravan or motorhome…
Series Connected Batteries
In this arrangement the positive of one battery is connected to the negative of the next. Each battery connected in series adds its VOLTAGE to the system. Two 12 volts batteries in SERIES will give you 24 volts. Four 6 volt batteries in series will also give you 24 volts. Series connecting does NOT increase the amp/hour capacity of the system. Two 100ah batteries connected in series will still equal 100ah – two hundred 100ah batteries connected in series is still 100ah.
In a series connected battery bank, every battery will discharge and charge at the same rate – it has to! The electrons flowing into or out of the system must flow through each battery. However, if an imbalance does occur, it will not resolve itself as the MOST charged battery will always stop the charging process (by raising its internal resistance). This is why it is never a good idea to tap 12 volts from the centre of a 24v series pair.
Parallel Connected Batteries
This setup has the positive of each battery connected together and the negative of each battery connected together. This has no effect on the system voltage. Two 12v batteries connected in parallel is still 12v – two hundred 12v batteries connected in parallel is still 12v. What does increase is the total amp/hour capacity. Two 100ah batteries connected in parallel is 200ah.
In a parallel connected battery bank each battery takes all the charge it needs – it and only it decides when it is full and stops accepting charge.
Ok – so which is best … series or parallel? It seems that the parallel camp has had a bit of bad press – lots of stories involving self-discharging and dead cells. I could not find any research that backed this up. From all that I could find it seems that most research and indeed the battery manufacturers themselves suggest that batteries in parallel will last longer than the same batteries connected in series.
So, what about this “one battery dragging the other down” issue?
Well the self-discharge rate for an AGM battery is about 5% per month. That is just 0.16% per day (in reality it is not linear process – but let’s keep it simple). This tiny self-discharge can be ignored by anybody who is using the batteries daily or even weekly. If you plan on storing the system you should take into consideration this self-discharge and be sure to keep a trickle charger on the batteries or at the very least, charge them fully once each month.
In most cases the connection method will be determined by the number of batteries required, physical size of each battery and the system voltage. In very large systems a combination of series and parallel will be required to get the voltage and capacity required. If you have the option, then I would recommend a parallel connection to provide for more even charging and discharging and ultimately longer battery life.
While we are squashing rumours, I am often asked if it is better (in a parallel connected system) to take the negative from one battery and the positive from another – (is it is shown in my drawing above). The short answer to this is that it makes absolutely no difference. Because of the size of the connecting cables (typically very large) they have close to no resistance and no voltage drop. It therefore does not matter which terminal you charge and discharge from in the slightest.
One other very important point (thanks Alan) is that in all cases the batteries must be of the same capacity and chemistry (ie never mix gel,AGM or flooded) – preferably the same age and from the same manufacturer. Failure to follow this rule will destroy batteries very quickly.
Just a reminder – I happily answer all questions (relating to caravan and motorhome electrics) posted on the site or by email. I also offer a design and consultancy service to people building or renovating their own motorhomes or caravans. Please email if you would like to know more.
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