If you're putting an electrical system into your van, then this email is for you! We see so many people using Split Charge or Voltage Sensitive Relays (VSR) on their van builds, but there is a way better option!
DON'T USE SPLIT CHARGE RELAYS!!!
Your engine is a valuable source of electrical power from the alternator, and even more so if you tend to move a lot. Most alternators have excess capacity that can be used to charge leisure batteries. Charging from your alternator becomes even more important when you have limited solar capacity and no shore power (electric hook up).
Split Charge Relays and Voltage Sensitive Relays (VSR) are very popular due to their low cost and relative ease of installation. However, as cheap as they are and as easy as they are to install there are a few points to consider, which may swing your decision. We think there are better options out there, and we’ll be talking about the reasons we say this below.
WHY NOT SPLIT CHARGE RELAYS?
With a split charge relay, you're essentially just wiring your leisure battery directly to your engine battery (or alternator) with an automated switch that turns on and off depending on whether your engine is running, so your leisure battery charges while the engine is running and does not drain your engine battery when the engine is not running. The biggest problem with this approach is that you’re connecting two dissimilar batteries together, even if they have similar chemistries.
The starter and leisure batteries are often at completely different states of charge. Although the starter battery has a very high current draw it’s all over in seconds and it charges back up quickly. The leisure battery is often depleted to a much lower state of charge
The capacities of the starter and leisure batteries are often dissimilar, especially if there are multiple leisure batteries in the bank. It’s not uncommon to have 85ah of starter battery and 200-400ah of leisure battery.
The chemistries of the starter and leisure batteries are sometimes slightly different, and sometimes VERY different. Even when mixing flooded lead-acid with AGM care should be taken. The charging voltage and current requirements may differ considerably. If your leisure battery is Lithium a split charge relay should NOT be considered.
One of the main reasons that we would not recommend using a split charge relay is that it does not regulate the voltage or current that you're getting from your engine battery (or alternator). On top of this, the voltage coming from the alternator can vary according to conditions (engine revs etc). And because the voltage coming out of the alternator can vary so much, you need to ensure that the voltage output from the alternator is OK for your battery system.
Even if your leisure battery is of a similar chemistry it’s often situated some distance from the starter battery, which can cause some voltage drop along the cable. Many people start off with smaller leisure battery banks and add to them later, but the problem is that often the cable installed at the start is appropriate for the smaller battery bank but not a larger one. In some cases, the leisure battery may never receive a high enough voltage to charge properly.
In general, simple relays are not able to deal with problems such as overheating batteries.
LiFePO4 and other Lithium derivatives: It goes without saying that a simple relay system is not a good idea.
WHAT SHOULD WE DO INSTEAD OF A SPLIT CHARGE RELAY?
If you want the most optimal way of charging your leisure battery, then we would HIGHLY recommend getting a DC/DC charger (also known as a B2B charger). Here are the main reasons that we would recommend using a DC/DC charger:
Most [good] DC/DC chargers have battery profiles built in that you can choose, which means that your battery will be charged in a way that is best for that battery chemistry.
Your battery will receive a constant voltage from the alternator as most DC/DC chargers regulate the voltage, even if you have [some] voltage drop on the cables from your engine battery. You would usually install a DC/DC charger close to the leisure batteries, meaning that the voltage drop from the charger to the battery is low. The DC/DC charger compensates for the voltage drop along the longer cable.
If your battery is charged in a way that's optimal for that chemistry, often this will increase the lifespan of your battery and it'll last longer.
A good DC/DC charger could protect your alternator. Because it can control the current draw from the alternator it’s possible to deal with situations where there is not that much spare capacity from your alternator.
A good DC/DC charger is able to deal with problems such as overheating batteries.
IS THERE EVER A CASE FOR A SPLIT CHARGE RELAY?
To be fair, there is a time & place for split charge and voltage-sensitive relays. Very very simple, single battery setups with low capacity requirements. And the starter battery and leisure battery must be the same chemistry (or similar voltage requirements). A simple van with occupants that just need lights, gas/diesel heater and mobile device charging. Even a single laptop would be OK for an hour or two a day. Add a second laptop? …. maybe. Add much more? maybe not.
However … we still think you should consider future-proofing your setup. Some of the DC/DC chargers are not that expensive.
If you have similar chemistries (voltage requirements) and have a good solar system and/or are often on shore power that could be another reason to go for a cheaper relay system.
OUR OWN EXPERIENCE
We used to use a simple split relay approach up until around 2007. We had a Nespresso coffee machine and a microwave at the time. To be fair, we hardly used the microwave - basically tried it once and it flattened the battery bank of 400ah quite quickly. We couldn’t do without our coffee though!
So we installed a Sterling B2B in 2007 and never looked back. It’s a great bit of kit.
