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 - touch wood!
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.
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.
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.
EHU (230v) charging - and the EBL ...
This is another grey area - similar to alternator charging. A standard lead acid charger will charge a lithium battery no problem. But in doing so it will be working at 100% until the battery is full - assuming the cabling will pass the full charge. So again, monitoring is important. This is especially true with later vans that have an Electroblock - EBL. An EBL contains a 230v charger and relays that allow alternator charging through to the leisure battery and the fridge. It also contains the 12v fused distribution to the van’s habitation circuits.
Earlier pre 95 vans don't have an EBL, and have a separate 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 give some charge, but not much.
If you have an EBL and are fitting a big lithium battery - 200ah or more, then the easiest way is to disable both the charger and the relays in the EBL, and just use the EBL for distribution. On most EBLs this is easily done - just unplug the 230v plug, and cut the yellow D+ wire from the alternator that operates the relays. Alternatively you can just remove 230v, and also the main battery cable from the engine battery and redirect the battery cable to your B2B. But both these methods mean that the 12v feed to the fridge while driving is also disabled. There are several scenarios to deal with this. If you want to retain this feature then you have to go inside the EBL and disable the main relay, but keep the fridge relay. How you do this depends on the model of the EBL. Another method is to disconnect both the D+ line, and the fridge 12v feed wire from the front of the EBL, and mount an external relay. Or you can just forget about running the fridge on 12v when driving, and run it on 230v while driving. Most owners who fit lithium will also have an inverter - you can connect the fridge 230v cable to the inverter. This is even easier if you are fitting an inverter charger to your van, which keeps the 230v circuits in the van live from battery/inverter when not on EHU, with auto changeover. Which method you use is down to what installation you choose. Personally, I believe that many owners install lithium because they want to run high power devices such as coffee makers, slow cookers, hair dryers and air fryers or similar. In these cases it is far more convenient to have the inverter connected to the vans 230v sockets so that they are live all the time, whether on EHU or inverter, with auto changeover. This is usually achieved by using an inverter charger such as the Victron Multiplus, but there are other methods, including some inexpensive ones using auto switches designed for generator backup - available from Amazon for less than 50 €£$. There is also the old fashioned way of manual switching, but with the price difference between manual and auto now so low, I can’t see the point in manual switching. I can provide details on this.
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.
So those are the main points to consider about charging lithium.
Types of lithium battery
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.
Basically there are 2 choices - DIY or ready made. A lithium battery is very different from a lead acid battery - a lead acid battery is "dumb" - it is pure metal, plastic and chemical, and it has no electronics in it at all. A lithium battery is quite different - a lithium cell is 3.2v, so to make a 12v battery you need 4 of these cells in series, which totals 12.8v. Lithium cells must not be either overcharged, or undercharged, so to achieve this, and to offer maximum safety and long life, lithium batteries need a BMS - Battery Management System. This is an electronics board that is connected to each of the 4 cells, and monitors them constantly. It balances the cells, and it cuts them off if the voltage falls below, or exceeds a preset value.
A DIY battery is where you buy the 4 cells yourself, and the BMS, and connect them all together yourself. A readymade battery is where a battery company has mounted all the cells and BMS inside a box and done all the work for you. And because the leisure vehicle industry is so used to lead acid batteries, the box they use looks like a traditional battery.
It used to be that DIY lithium was cheaper than readymade, but nowadays this is not the case, especially for batteries of around 200ah or less. At the time of writing in Summer 2023, Renogy are selling a 100ah battery with Bluetooth for £420. But if you want a really big battery - say 600ah or more, it is usually cheaper to go the DIY route.
The differences between DIY and readymade are enough for an article on its own - but generally DIY is for enthusiasts, and readymade is better for non tech owners who just want to plug and play.
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, and is currently about 3 or 4 times the price. 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. The worst reason to upgrade is simply because it is the "latest thing".
You have to define your power needs, and then design the system around this
