Too busy for a long article? Can you just “drop in” a lithium battery into an unmodified classic Hymer? Basically yes you can, as long as you keep an eye on it, and the best way to do this is to choose a "smart" lithium battery that comes with a bluetooth app for your phone or tablet that shows you exactly what is happening to the battery in real time. Then you are in control. There is really no point in installing a "dumb" lithium battery - unless you are quite technical and have other means of monitoring and testing.
For the technical discussion .... read on.
Can you just “drop in” a lithium battery into an unmodified classic Hymer? I am afraid that there is no definitive black and white answer. I asked 3 leading lithium leisure battery manufacturers this question. One said yes, the other said their advice was to always fit a battery to battery charger, and the other didn’t reply!
So lets take each of these charging sources in turn, and try and assess their suitability for charging lithium.
The main issue is that lithium has different charging parameters to normal batteries, and the implied danger is that charging devices might get overworked, because a lithium battery can usually take all the charge presented to it. This could result, for example, in components only originally designed for charging a standard 100ah leisure battery with around 20a of charge, to deliver much more than that when a lithium battery is installed.
Alternator charging
The standard pre 95 classic, Fiat or Merc has an alternator of around 50 to 70a, and cabling between the engine battery and the leisure battery, via the split charge relay behind the fuse panel, of around 10mm2 copper, and intended to handle around 20a. Hymer fitted a 70a relay, but it is important to remember that this was over specification by Hymer so that the relay would last for ever - not to actually handle 70a. But the bottom line here is that you are starting with a charging circuit with a design capacity of around 20a. But this isn’t self regulating - it doesn’t limit itself automatically - if asked to do more, it will, but then every component in the chain of the circuit will be working harder than before and problems can occur. In practice, if an original charging circuit is used, then it needs to be monitored for the first few weeks and months. The limiting factor is usually cable thickness, but connections can also be a problem, especially old ones that may be corroded or tired.
But some owners can and do charge lithium directly from the alternator - by upgrading cables and relays. Bt these owners are usually technically minded, and have metering and monitoring in place to ensure that every component in the chain is solid, and that the alternator isn’t being asked to work too hard. You have to keep and eye on it, and have the metering in place to monitor it.
The other worry that you read about online is that a charging system designed for lead acid batts is not suitable for lithium. This is actually not really a problem. A lead acid charging circuit will deliver around 13.8v maybe a bit more. But the limit for lithium is 14.6v, and even if this voltage was reached for some reason, the lithium bat's BMS would kick in and disconnect the charge. And if you look at the cgarge vs voltage chart for lithium batts you will see that even a charge that goes no higher than 13.8v will still result in a 95% charge. And in practice, with most vans that have lithium will also have solar, with a solar controller that has a lithium profile, solar will take care of the remaining 5%.
What many forget is that lithium batts are not dumb like lead acid, they have brains - the BMS, Battery Management System, and the job of the BMS is to prevent abuse. So the BMS will simply switch off the battery if the voltage or current falls outside of preset safety values. So in reality it is actually almost impossible to damage a lithium batt by giving it wrong voltages - it will just shut down.
But when it comes to alternator charging the simple solution for most owners is to fit a battery to battery charger - a b2b. This is a device that sits between the engine battery and the leisure battery. It has two main functions - to limit the amount of strain put on the alternator, and to present the leisure battery with a near perfect charging profile. A b2b is easy to fit - it has just 3 connections - to the engine battery, to the leisure battery, and the D+ alternator control line. (some b2bs don’t even need D+ - they are auto sensing). The only other issue for classic Hymer owners when fitting a b2b is that the original Hymer split charge relay (discussed above) needs disabling. This is easily done by removing the original relay from the fuse box. All other original cabling is left intact, which preserves the original function of 12v fridge operation while driving. Then a new cable has to be installed from the engine battery to the b2b, and from the b2b to the new lithium battery. The size of this cable is dictated by the size of the chosen b2b. And the size of the b2b is dictated not by the size of the new battery, but by the max rating of the alternator. The general sensible rule is 50% of the alternator rating - for te vast majority this will be 30a.
If fitting a b2b which needs D+ switching, then the original D+ line to the original relays in the fuse box has to be identified and tapped into, and a new wire run to the b2b.
In the case of classic Fiat vans, where the engine battery is under the front bonnet, a longer cable to the b2b is usually needed, and the D+ line can be run alongside it - the original D+ wire is usually able to be found in the area of the engine battery.
In the case of classic Mercedes, with the double battery box next to the drivers seat access is easier - the b2b can be easily connected to the engine battery and from there to wherever the new lithium battery is located. An easy way to obtain a D+ signal in the Mercedes battery box is to open the trunking that leads across the bottom edge of the door (if fitted) and locate the 12v power feed to the fridge, and carefully split the cable and tap into the fridge feed. The 12v fridge feed is relay switched by the D+ in the fuse box, so is in itself a D+ proxy - this method just makes it easier. Please note this is a tip for certain types of install, not a mandatory requirement. Most modern b2b chargers, such as Victron orion, have voltage sensing and don't need D+.
The other factor that needs thinking about when considering alternator charging, is just how much you want. There is a trend with lithium batteries to “go large” and fit batteries of 200ah or more. So you have to think about just how much driving time you will need the next day, in addition to whatever you get from solar, to replace the power you have recently used. With most classics having around 60 to 80 amp alternators, and a sensible limit of around 50% of this, then that means no more than about 30 to 40 amps per hour of driving will be available. This is usually enough for most owners, but if you want more, then the only alternative is to fit a bigger alternator. But before you do, consider this ...
Solar keeps working all the time, and many vans with lithium also have solar, and those that don't should have! And modern solar panels are big, and cheap. In 2025 you can get a 425w black panel that is 1700 x 1100mm and will fit perfectly on the front of most Hymer roofs. And if you have such a panel, or even 2 or 300 watts of other panels further back - these panels don't stop working while you are driving. On a sunny day in summer I have regularly seen 70a of charge while driving - 40a from the alternator and another 30 from the solar! So factor in your solar ad your driving habits (winter/summer) before you go to the expense and hassle of a bigger alternator. In my experience a bigger alternator is rarely justified if thought out properly. It depends on your lifestyle and intended power use.
EHU (230v) charging
Pre 95 vans have a separate 230v charger. Most owners have replaced the original separate charger, so what charger you now have dictates what mods if any are needed. The original factory charger, if you still have it, will work with lithium, but will not be optimum. But if you do need to use it, you can.
But most owners fitting lithium will opt for a new charger. The Victron Blue Smart IP22 30a is ideal.
When fitting a new more powerful charger for lithium, the next question will be whether to use the original Hymer cabling or not. This is particularly relevant on vans that have the charger in the back and the battery in the front, it is tempting to use the old cable because running a new one is a big job.
You can use the old cable - it will handle 30a, but you have to take into account that the original charger was only 10a, and the 12v cable from the old charger to the front of the van goes to the fuse box, and then via a 16a fuse, and then to the battery. Not only will the fuse blow if you fit a 30a charger in the back, but the length of cable, with old and possibly corroded connections along the way, will lead to an unacceptable voltage drop - ie won't charge the battery optimally.
So what do you do in this situation? And remember we are only talking about vans with the charger in the back and battery in the front. You have 2 options. The first is to move the charger to the front and connect directly to the battery. This means you will have to supply the charger with a new 230v supply. How easy this is depends on your van layout and your DIY skills. The second is to use the original cable, but disconnect it from the fuse box and run it directly to the battery. This takes out the problem of the original fuse and the old connections in the fuse box, and it also shortens the cable run considerably. I have performed this mod on many vans and it works well. But there are different tricks to identify the cable and do this, depending on the van. Anybody wanting more info on this can contact me direct.
On vans with the charger already in the front, things are much simpler, but you still have the problem of the original charger cable going via the fuse box. So in these cases it is usually best to simply run a new cable, and leave the old charger cable unused and safely insulated and tucked away. But it can be used if you are willing to put a bit of extra work in - it can be disconnected from the fuse box and re-routed directly to the battery. Again I have hints on this if needed.
Solar charging
This is probably the easiest of all. Solar power is free from the sun, and is the only charge source that is available “off grid”, so is ideal for wild camping. Solar panels and MPPT charging are now so cheap, and powerful, that if you are fitting lithium batteries it makes sense to fit as much solar power as your roof will easily take. On most vans this will be between 200 and 400w. This means that on a summers day, you can reasonably expect to replace between 80 amp hours (200w panel) and 160 amp hours (400w panel) on a sunny summer day, based on 6 hours of sunshine. But in winter the figures can be as low as 20% of this because the sun is much lower and the days shorter in winter. This is no problem if you don’t use your van in winter, but for full timers it is a major factor in your off grid capability. Full timers who want to be off grid as much as possible need to fit as much solar as they can. This goes for any battery system, not just lithium. A common misconception is that a 200w solar panel is a constant - it is not! In December it is a 40w panel! And that is assuming that the sun shines in December.
Technically, modern solar MPPT controllers usually have a lithium profile, but you can still use those that do not. If the controller has user definable settings, just set everything to 14.2v. If this isn’t possible, then just leave it set to the lead acid profile, it won’t do any harm.
The result of all or some of this work is that the new lithium battery will have some or all charge sources (EHU, engine, solar) connected directly to it. But power still needs to get from the battery to the habitation circuits - so the original Hymer cables are left in place - black positive and brown negative. These feed direct to the fuse box, and then to the van.
If you are fitting high power devices such as an inverter, then this should be connected directly to the battery via suitable fusing. More complex installations may use busbars or connection posts.
Types of lithium battery - what to buy.
The next subject to discuss is "what type of lithium battery"? All lithium batteries for the leisure vehicle industry are lithium iron phosphate - known as lifepo4. These are the safest type.
Prices are now so low that lithium batteries are hardly more expensive than lead acid. The choice nowadays is do you opt for "dumb" lithium or "smart" lithium. Smart means the battery has a bluetooth BMS that talks to an app or display. In the motorhome context, the choice is a no brainer - always buy smart. Having an app in your hand which tells you exactly what is going on in real time (see screen grab below) is the killer app of modern battery technology. It is daft to fit a dumb lithium battery to a motorhome. But for those that have fitted dumb lithium, all is not lost, because you can make a dumb system smart by adding a Victron Smart Shunt (or cheaper chinese version) which will then offer you the same valuable info as a smart system. But note that if you shop around, you can now buy smart lithium 12v batteries for roughly the same price as dumb ones - Epever and Fogstar from Bimble are 2 solid recommendations. Note that when browsing 12v lithium leisure batteries online, if it doesn't specifically say "Bluetooth BMS", then it will be dumb. There is a huge amount of old stock flying around, and also a lot of cheap chinese lithium leisure batteries. Chinese batteries from either AliExpress, or UK/EU suppliers and importers are everywhere now. There is great debate about quality. There are Youtubers who cut open cheap chinese batteries and dissect them and analyse at length. I have even done it myself. My conclusion - and I am by no means the last word on this - it is just based on experience, is that there is nothing wrong with cheap no name chinese lifepo4 batteries - as long as they are smart bluetooth, and you have to be careful with the ad small print. There are lots of dumb batteries around - used in golf carts and fishing boats (trolling motors) and many other applications that don't need smart - but for a motorhome owner, smart is essential, because having all that info in your hand is a game changer for van life - it puts you in control - no more unwelcome surprises of the lights going dim half way through the weekend.
To my mind, the choice comes down to whether you want UK support and warranty - if you do, you have Bimble, Fogstar and several others, and you will pay a bit extra. Renogy and Ecoworthy also deserve a mention - their support is not quite as good, but it does exist if you push. But the unknown suppliers on AliExpress, you will get little or no support.
But little or no support isn't as bad as it sounds because these batteries are pretty reliable. The reason for this is that unlike lead acid batteries, which are easy to abuse, and fail as a result, it is actually very difficult to damage a lithium battery because they have a computer brain - the BMS - protecting it. If you abuse it, it just shuts down.
So basically cheap lithium smart batteries are fine for motorhome use. By all means pay a bit extra, but if budget is tight, you are usually OK.
But there is one area where it pays to go big on quality and that is high power. Big lithium at a reasonable price means that things that were only dreamed about in the past are now possible - I am talking about high power - microwaves, airfryers, hair dryers, coffee machines, toasters, even kettles, these can all be powered with a big lithium battery through an inverter. Putting aside that the installation has to be solid and well installed, and that any power you use for these luxuries has to be replaced somehow .... but just concentrating on the battery .... if you power a 1500w device that will be drawing over 100amps from the battery. That is a lot of power, so now quality does become a factor. Big power needs lots of copper, and generates heat, so build quality is important. So if you are planning big power then probably best to buy a battery at the top end of the scale. Reputable suppliers will publish detailed specs, and 200amp continuous discharge is easy to find.
But having said all that, as long as you have done things right in terms of cable spec, there is nothing wrong with a 20 minute airfryer session from a cheaper battery. Even the no name suppliers don't want warranty returns and bad online reviews.
many owners have no idea about power - they are used to just plugging anything in at home and it works - for all intents and purposes power at home is limitless. But it is completely different in a van. It takes a LOT of 12v power to convert to 230v.
So here is a list of common appliances to guide you.
Low power, no problem under 50 watts- phones, pads, any small battery appliance -shavers tec.
Low-ish power - 50 to 200w - TVs, laptops, ebike chargers.
Medium power - 200w to 800w - small coffee machines, small kitchen appliances such as whisks.
High power - 800w to 1500w. Small airfryers, small kettles, microwaves, big coffee machines, hairdryers. Basically anything that generates heat - check the label for the watts, divide by 12, that is how many amps at 12v you need.
Air conditioning is a special case and a separate conversation.
Finally .... do you really need it?
There is also one more big thing to think about - and that is - “do you really need it”??!!
There is nothing special about lithium - it is exactly the same volts and amps as any other battery. So you have to really identify the need.
The three main advantages of lithium are power density, long life and better information. Power density means you can get more power out of the same size - so a rough example would be if you only have space for 1 traditional 100ah leisure battery, and you don't want or can't relocate the battery, then you would get roughly double the power from the same space.
Long life is exactly as it sounds - they last longer. In regular use a standard leisure battery is only good for a few hundred cycles of charge and discharge which in average use means they need replacing every 2, 3 or 4 years. Lithium batts are good for thousands of cycles. The jury is still out because it's new technology but it is generally accepted that a decent lithium battery setup should last decades. So if you use your van a lot and intend to carry on doing so for years, then the financial deal alone can justify it.
But the third reason, better information, is for me, the best reason by far - better monitoring and information. The latest lithium batts come with bluetooth monitoring, and a phone app that displays accurate battery information. This tells you exactly how much power is left in the battery, how much charge you are getting (from all sources), and how much power you are using. Basically it is an accurate battery fuel gauge. After a few days of use you soon get a feel for your battery system, and can plan your activities accordingly. This is particularly useful for those who like to be off-grid as much as possible, or for those who may need to run devices that potentially use a lot of power, such as coffee machines, ebike chargers etc. You soon get to know exactly how much power these devices use, and how much charge you need to replace that power used - from solar etc. It is incredibly useful and gives you complete control, with no nasty surprises. With a standard lead acid battery - it is quite difficult (but not impossible) to get this sort of accurate information. Having used these Bluetooth lithium battery systems for over a year now, for me, it is by far the best reason to do the upgrade.
But none of that is to say that lead acid batteries are old hat or obsolete - they are not. A lead acid battery with a good solar system can easily provide enough power for normal summer holiday use. One of the best reasons to upgrade is when your existing system simply isn't giving you the power you need.
But now that lithium is only slightly more expensive than lead acid, it should be considered. And for the hobbyist and technical enthusiast, a lithium upgrade is a fascinating project.
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