Adding more leisure battery to a classic Hymer ... things to consider

One of the commonest upgrades to the electrical system is to add more battery capacity, but before you do, there are questions to answer and things to consider before parting with your money. This article is not intended as a complete "how to"  but rather a discussion of all the things to consider and the possible pitfalls.

Do you really need it?

One of the first things to consider is to make sure that you actually do need more amp hours, and to answer this you first need to know that your existing battery is really up to scratch. It may just be that your existing battery is so poor that a new single battery is all you need to solve your problems.

Unfortunately testing a battery is not a simple task, unless you have it tested by anybody or any business who has a modern digital battery tester. The only way to test a battery manually is to use a multimeter and a known load. If you can, find somebody with a digital battery tester - most garages have one these days.

So have your existing battery properly tested, and if it is found to be defective or just worn out, replace it with a new single battery and see what a difference it makes. It may just be that a single new battery will perform so much better that you will no longer need to consider adding a second battery.

There is also a huge extra benefit to this approach, because if you do decide to go ahead and add an extra battery, you can add one of the same make and capacity, which is very important. You just go back to where you bought the first one and order an identical one.

Batteries in parallel

The reason for this is that when connecting batteries in parallel, they two batteries should be identical. This is quite an important rule, and if followed will give you the maximum power for your money.

However I have found from some experience that it is not absolutely carved in stone and it is wrong to think that if you break this rule you will get nothing but grief and failure.

The reason the rule exists in principle, is that if you put dissimilar batteries in parallel, then the weakest of the pair will pull the other one down, over time, to its own capacity and performance. So put a good and a bad battery together and the good will suck the juice out of the bad! More or less.

So the situation to definitely avoid is putting a brand new battery in parallel with a bad or old one.

However, it is often possible to put dissimilar batteries in parallel and get some increase in total power. The batteries should be as similar as possible - similar age, similar amp hours and similar chemistry - ie don't mix an AGM leisure battery with a starter battery. You must approach this with common sense and a willingness to experiment. 

Now I will probably attract some criticism for that last paragraph, but I am unwilling to say that you must NEVER mix batteries unless they are totally identical - there are some circumstances when you can. However, in most classic motorhome scenarios most owners will bite the bullet and buy 2 identical. If in doubt - buy new.

Where to put an extra battery

So, having established that you are going to fit a second battery, let's move on to the next problem. Where to put it. Squeezing more battery power into a motorhome is often more a case of finding space than anything else.

In purely classic Hymer terms - The Mercs have a battery box in the floor by the driver's seat that contains both the leisure battery (LB) and the vehicle engine battery (VB). The Fiat family vans usually have the VB under the bonnet, and the LB under one of the front seats. To fit an extra LB can be anything from a doddle to a nightmare. Here are a few examples.

The easiest vans are those that have a dinette or a bench seat that has empty space under it, as close to the original LB as possible. These present a void that is perfect for an extra battery.

However some other vans are the opposite - the S700 and the B694 are good examples - there is a bench behind the driver's seat, but it is full of water tank! There is literally nowhere in the front of the van to fit an extra battery. The only place possible is under the rear bed, and that means a serious amount of cabling. 

Evey van is different, so you just have to find a battery shaped space somewhere.

It can get so difficult that many owners give up at this stage. However there are alternatives. One of these is to forget about adding a second battery, and to maximise the potential of the existing LB space. This might mean finding a slightly bigger really high quality battery, such as an Odyssey or a Northstar, which will maximise the potential of the space - you might not get double the capacity, but you might get to 50% more.

Another more radical route, especially for full timers or power users, is to make the jump from bottled gas to underslung LPG tank, which gives you an empty gas locker for batteries (and other stuff!). This is quite an expensive option, but it does make sense for many - myself included.

But for many, especially owners of the bigger 6 and 7 series vans, there is only one viable option, and that is under the rear bed. This poses other problems, but none which are insurmountable. 

It might seem that the obvious way would be to have a new battery in the front original position, and the second one in the back, connected by long leads. This is valid, but I think there is a better way, and that is to locate both batteries at the rear, or better still, splash out and buy a high quality single battery of double the capacity. A single bigger battery is always the better option if you can, than two in parallel.

The problem with moving the battery to the back of the van, is that all the wires that are connected to the original battery are all at the front - so you might think you are faced with a big rewiring job, but you are not if you approach it the right way.

Spit your dummy out ...

The way to do it is to mount 2 terminals, or busbars, where the original battery used to be, and connect everything that used to be connected to the original battery, to these. They basically represent the original battery - or to put it another way - a dummy battery. So everything remains the same, no rewiring needed, just connect all the original wires as they were. Then all you have to do is to locate the new battery(ies) wherever you can, and then run two wires, good and thick, from the new battery to the dummy terminals.

Using this method, you can now mount a battery anywhere in the van you like, as long as you adhere to basic common sense safety measures. 

Cables cables cables

At this point you would normally expect quite a long discussion on what cable to use, and cable loss. This is a topic that causes some confusion. When working with 12v, unlike 220v, the losses introduced by cable are quite important, as well as its power carrying capacity. For the purposes of this article I am going to simplify it somewhat. The cable you use to connect your extra battery capacity, if mounted remotely - ie at the back of the van - should be capable of carrying between 50 and 100 amps with no more than point one of a volt drop (0.1v) - you can push this maybe to 0.15, but not much more. There are tables of losses and cable sizes all over the internet, and of course you can ask on our FB group - Classic Hymers Technical. If you really want it simple, then you should be OK with copper cables the thickness of your little finger - but copper is expensive, so if you can, take the scientific approach!

But if your batteries are in a convenient location close to the original, then thick cables is less of an issue, unless you have a high power application in mind. So up to around 2m away, cable around 4 or 5mm should do, but again, use the thickest you can afford.

My favourite cable utility is here https://www.12voltplanet.co.uk/cable-sizing-selection.html

Cable routing

If you do end up running cable from the front of the van to the back - you have two choices - through the cupboards, or down through the floor along the chassis, and back up through the floor. Every van is different, but generally speaking it is easier (although dirtier!) to go along the chassis. There is plenty of room under there, and plenty of attachment points. Again, I can't go into chapter and verse about cable routing and safety - all I can say is that unless you are confident that you can run and protect your cables and protect against vibration, short circuit and weather, then perhaps you shouldn't be doing the job yourself.

Another option to save copper, is to just run a single positive (red) cable, and to use chassis for the negative. In this case you would need a very solid bare metal to bare metal chassi connection at the remote battery end. At the front end you would need a solid connection to the VB neg, as this is already connected to chassis by a thick cable. Some purists will not agree with this, but I have used both methods with no problems. Again, if you don't understand what I mean by chassis return, you should come and ask more questions, do more research.

Fuses are essential ...

Another safety issue is fusing. If you are running a long cable to extra batteries, then you must protect that cable against being shorted in a crash. If a live battery cable is shorted to ground as a result of a crash, the battery will immediately discharge over 1000amps in milliseconds, which has the potential to cause a spark and a fire. To guard against this there should be fuses at both ends of a long battery cable run. The easiest to use are "mega fuses" - see photo - usually 100amps is the right size. These should be located as close to the battery as possible.

Parallel Series Parallel Series

Finally, let's talk about batteries in parallel. Batteries in parallel means two (near identical) batteries connected "in parallel", which means the pos and neg terminals of both batteries are connected to each other. Batteries in parallel remain at the same voltage, but double the power. So two 100ah 12v batteries connected in parallel will give 200ah at 12v. Batteries in series double the voltage at the same power, so two 100ah 6v batteries in series would give 100ah at 12v. batteries in series are rarely used in motorhomes, so we will leave it at that.

The next thing you will see if you ask the question online, is that batteries in parallel must be connected as a set - ie you should take power from them, and deliver charge to them via the pos on one battery and the neg of the second, or vice versa. See diagram below. Again, this is a purists view, although completely correct in theory. The reasoning is that this will present the two batteries as one. However in practice, the differences are negligible, and certainly not making the major wiring changes that may be necessary in a motorhome installation. If your two batteries are together and easily wired, then by all means, but if you are faced with major rewiring, then don't bother.

The simplest check you need to do with batteries in parallel is this - with a digital multimeter, check the voltage of both batteries. They should be identical, to two decimal places - ie if one says 12.86v then so should the other.

Treat the system as a whole ... Chargers and Alternators

Next - you need to think about your overall system in the context of bigger batteries. When you upgrade one part of a 12v system you need to think about how it affects all the other parts. The most important of these is charging.

A 10 amp charger should charge a 100amp hour battery in ten hours - again simple arithmetic. So if you up the capacity of the battery to 200 amp hours, then a 10 amp charger will take 20 hours. This means that you could be faced with a situation where an overnight charge from EHU may not have had enough time to recharge your batteries fully. It's an overly simple example, but it illustrates the point.

It gets a bit more complicated than that as well .... a lead acid battery needs to be charged by a charger that is capable of delivering at least 10% - 20% of its capacity. That means 10 - 20 amps for a 100ah battery and 20 - 40amps for 200ah. Personally I think around 15% is about right.

What it does mean is that the original blue box factory fitted dumb Hymer charger (usually 10 amps) really ought to be replaced, especially if you are fitting 200ah batteries.

If you are going to the expense of new 200ah batteries, then you should definitely consider installing a new mains charger of at least 20a, preferably more. 220v chargers come in all shapes, sizes and qualities. The best and most popular is Victron Blue - never known a bad one, plus they have the advantage of being fanless. You don't really want a fan cooled charger running all night and keeping you awake.

The next thing to consider is charging while driving, which is a very important source of power - you want to arrive at your destination with a fully charged set of leisure batteries.

In a standard pre 95 classic Hymer, charge is diverted from the engine alternator to the LB via a relay located behind the fuse box. (in later vans, fitted with an Elektroblock the relay is in here). This relay, and its associated wiring, is designed for the 10 to 20% rule - ie to charge a standard single 100ah LB, it needs roughly 10 to 15 amps from the alternator while driving. This will not automatically double if you double the battery capacity to 200ah. The max amount of charge able to be delivered by the alternator is unfortunately quite a complicated mix of wiring capacity, relay capacity, max capacity of the alternator and other requirements of the base chassis - ie the headlights and state of charge of the starter battery. There are many variables, so you just can't simply expect double.

Some of the early classics only had a 60 amp alternator, most later models had 80 amp or more. In most cases, doubling the battery to 200ah should not result in any major problems - but don't expect the "Strom" meter on the panel, the one that shows you how much charge your battery is getting, to go hard over and show 20 amps of charge from the engine - it might, occasionally, but it won't stay there! So, like the charger situation outlined above, you may be faced with a situation where even a 4 or 6 hour drive may not be enough to charge a 200ah battery from empty. If this is the case, then there are solutions available - usually all you need is a bigger capacity relay and wiring, but occasionally you might (rarely!) need a bigger alternator, especially if your van is an older 80s model, and you desire a really big leisure battery setup.

The same applies to solar - but every solar installation is different, as is the weather, so there are no hard and fast rules about solar, other than the fact that if you are a power user, you need as much solar as you can. For some owners, solar is massively important and their main source of power, for others it's just a top up and trickle charge - every van owner is different.

Lithium

A note on lithium - lithium batteries are coming down in price every year, and are now standard in many new vans. They offer roughly about double the "power density" of lead acid, which means that you can get twice the power from the same space. However, you can't just drop lithium batteries into a classic Hymer - you have to look at the system as a whole. That is not to put you off lithium, just to say that they are still quite expensive, and need careful design and consideration.

Conclusion ...

The takeaway point I am trying to make here, is that the original classic Hymer is designed around a single 100ah leisure battery, based on 1980s technology. You can't just throw in an extra battery and expect to have double power, all the time. That is not to say you can't "just do it", but you should at least be aware of the potential pitfalls.

The good news is that other than finding the space for extra battery in some vans, upgrading and modifying the electrics in a classic Hymer is, compared with many modern vans, relatively easy. They generally have space to work, and the work can be hidden reasonably nicely and a good job made with only basic DIY skills.

This is how batteries in parallel should be connected - in theory

But you can connect them this way if it means avoiding major wiring changes, and on completion, both batteries have the same voltage to 2 decimal places.

This is a Merc battery box with the LB removed and replaced by a dummy. All the connections that used to go to the LB are now connected to the dummy - pos on the left, neg on the right, and the remote LB is now connected to the dummy. In this case, chassis is being used for the neg return, and the red wire, centre lower, goes to the relocated LB.

This is the same setup, but now with a protecting mega fuse added (the small square yellow thing!), and a Victron Cyrix relay replacing the original Hymer relay, to deliver more alternator charge to a bigger LB setup at the rear of the van.

This (work in progress) photo shows an Odyssey PC1800 220ah battery fitting snugly in a repurposed gas locker. The blue box is a Victron inverter charger.