We are pretty sure we want lithium batteries (LiFePO4) in Chip-Chip. Mostly because we don’t have a lot of space, and as lithiums can be discharged to about 20%, they will give us more power in less space and with a lot less weight.
But lithiums are fairly sensitive, so it is necessary to carefully think the electrical system through before installing them. Our first thought was to buy a “plug-and-play ” lithium setup, which includes a BMS system. This seems like the easiest solution, but the more we learned about lithiums, the more uncomfortable it made us not knowing the details of the system. Then we considered building our own battery from individual cells. This is the cheapest way, and seems to be quiet common in the off-grid community, but it is also the most difficult and time consuming option. So in the end we have chosen to go with victron batteries, which have build-in balancers between the individual cells, and victron provides a whole range of products to complete the setup.
But installing lithiums is still not easy. So here I will try to outline our preliminary plans for the setup.
In our current setup we have a house bank and a starter battery. These are both charged by the alternator, and optionally by an AC charger. The two banks are separated by a cyrix battery combiner. So it’s a pretty simple setup. To keep an eye on the system we have recently installed the Victron BMV-702 battery monitor. A great piece of equipment!
So which changes are needed to be able to safely use lithium batteries? After reading a lot about batteries, here are some of the main concerns we have right now.
- Firstly, want to make sure that the lithium batteries are not destroying our alternator. The boat still has the original Volvo Penta MD2B, so our alternator has no external regulator with a “lithium” bottom. Lithium batteries can be charged with a high current for most of the charging cycle, while the alternator is not made for providing a continuously high load. It is therefore necessary to be able to limit the charging current to prevent the alternator from damaging itself.
- Secondly, we want to make sure that we can control the current from the alternator. The lithium battery needs a charge current of 14.0 to 14.4 V. We have measured voltages of up to 14.8 V from our alternator, so that is not great.
- Thirdly, the lithiums will be damaged if they are discharged too deeply or over charged. So we need to be able to stop charging once the maximum is reached.
- And finally, a lithium battery is made of four cells, which are not automatically balanced. They will be charged slightly differently and thus end up slightly out of balance at the end of the charging cycle. A lithium battery therefore has to have a system which balances the cells.
It has taken us quiet a while to figure out what to look out for, and we will of course still want a professional opinion before buying anything. But so far, our solution is to buy the victron lithium battery, which has a build-in balancer (Point #4), and connect it to the victron BMS 12/200 (BMS is short for Battery Management System). The BMS 12/200 is able to control the charging current from the alternator, thereby protecting the alternator (Point #1), it keeps voltage fluctuations down and makes sure that the charging current does not become too high (Point #2) and also stops the charging once the battery reaches a maximum (Point #3). Additionally, the BMS 12/200 separates the two banks, making sure that the lithium battery neither charges nor decharges the starter battery.
To get an overview, we have made a diagram of what we think our new electrical system should look like. Simple, right? If anybody thinks we made mistakes here, please let us know!