HLPdata has developed a system for lithium batteries in boats. It is suitable for the normally handy boat-owner who want to replace their lead batteries themselves at a significantly lower cost than the big manufacturers can offer.

When changing your lead batteries in the boat to lithium batteries, there are some things that control what type of batteries you should choose: safety, lifetime, price and time to recharge.

If you start with safety then you should choose a technology that is stable and that is almost impossible to get to catch fire or "explode". This leads us to LiFePO4 batteries, or LFP as they are often called. With a reasonable price, a lifespan of thousands of cycles and the possibility of recharging quickly, we do not discuss other technologies here, but rather sum up that LFP is the technology that is best suited for a boat today. The only thing that is not at the top is energy density, which is why they are not used in phones or aircrafts, for example. In a boat, the weight is not as crucial. But compared to lead batteries they are considerably lighter.

But then comes the "problem". It does not work to just replace the lead batteries straight off, no matter how much advertising promises. LFP batteries are easy to destroy. Emptying them completely will destroy them immediately. If they are overcharged, they are destroyed, though more slowly. Likewise if you charge them with high current when they are too cold. Or use them so hard that they get too hot. And this is where HLPdata's BMS4S system comes in. It ensures that none of the above happens by monitoring the electrical system and automatically stop charging, shutting down consumers and/or alerting when something needs to be done. However, the only thing that normally happens is that a cell in the battery can become completely empty, but before that happens BMS4S automatically switches off all consumers. Or when the first LFP cell has been fully charged, then the system turns off the alternator and/or any battery charger to prevent overcharging. The system can also monitor the temperature of the batteries and the alternator to prevent problems. And the system also constantly estimates the actual charge status (SoC).

When you start, the electrical system in the boat looks something like this:

You often have a starter battery and a house battery. These are kept separate so that the starter battery is not used by the boat's consumers, and they are charged via a charging distributor or a relay that switches on when the engine is running. If you have bow thruster then you often also have a battery for it, placed in the bow of the boat. All batteries, of course, have switches and fuses connected.


If you want to make the simplest possible installation of LFP cells, then you simply replace the main battery with cells of your preferred size. If you want a really big battery bank, we recommend that you connect several smaller cells in parallel instead of individually larger cells as smaller cells are more resistant to all movements that happens in a boat.

Now connect the BMS4S according to its instruction against the LFP cells (dark red cables) buzzer / alarm (purple cables) and 12 (13) V power supply (light red and blue cable). Now you need to do a couple of modifications of the existing electrical system:

1: First move the alternator (yellow) voltage sense cable to BMS4S. If your alternator do not have a voltage sense cable you will need to connect a relay over the power feeding of the rotor, and let BMS4S control the relay. But this means probably that the alternator is fairly old. it might then be a good idea to replace it with one that can produce some more amps since the LFP cells then will recharge must faster than what led batteries do. If you still want to keep the old alternator, the user manual describes what you need to do.

2: Then connect the bi-stable relay between the LFP cells and the main panel so that BMS4S can disconnect all power consumers in case of a low voltage event, and connect it to BMS4S (green cables).

You should also test if the alternator itself prevents overheating. After running it loaded some half hour touch it with a wet finger to see if it boils, i.e. if it is more than 100 degrees celsius. If so you can let BMS4S supervise the temperature, and adjust its output-current to maintain an acceptable temperature. Consult the user manual to find out how. Many modern regulators though have temperature-supervision built-in.

When the installation is done, you should start by charging the four cells fully to the exact same charge level, which is important for the life length of the LFP cells. This is most easily done by charging the cells one by one with the 3.6 V charger (you can of course do this at home before installation, and then you can alternatively connect all cells in parallell and charge them together).

Then just turn the power on and leave port. As the battery is fully charged, the BMS4S will immediately notice this and switch off the alternator for 4 hours (adjustable) to prevent overcharging. If you then have too many lay-days and the LFPs are running low, BMS4S will switch the bistable relay so that the boat's power-consumers become powerless. As soon as you start the engine and the voltage start ro rise the power will be switched back on automatically.

But there are some disadvantages with this setup. Since the LFP bank will keep the voltage down below 13.8 V during most of the charging time, the led batteries with not get enough charging to be fully charged, which will damage them over time. There are two ways to solve this:

1: If you want to keep the led-battery as starter you can connect a DCDC-charger between the alternator and the led-battery. It will compensate for the low voltage from the alternator to the led battery during charging. And it will be without current when the engine is off, so it will not discharge the main battery. Here is a picture of how it can be connected.

2: Keep the led-battery only as a backup. This is what we think is the best and simplest solution. Remove the power distributor and connect the two battery bank together:

Now the lead batteries are only kept for backup and security, and should be kept disconnected by the switch during normal usage. All equipment will then be powered by the LFP cells. But keep in mind that electric motors is a source of a lot of disturbances, so ideally they should have an own connection to the battery, or as in the picture, a very short common wire in comparison to the distance to the motors. This to allow the battery to absorb these disturbances. And now, since the lead batteries not are used, it is enough that you every now and then switch them on and get some charging from the LFP cells and the alternator (and other charging source) to keep them in shape. You can of course automatically switch on a simple relay so the led-battery is always connected when the engine is running. This relay is then connected to the D+ terminal on the alternator.

You can connect to BMS4S with your mobile phone at any time and read an estimated charge status, battery voltage and voltage on each individual cell. It is when the first cell reaches a limit and it it risks being damaged that BMS4S responds to. This means that if there is an imbalance between the cells, they will not be damaged. Since the manufacturers recommend charging the cells fully after each longer break, it is a good habit to charge them with the 3.6 V charger before start of each season. Then you keep them guaranteed well-balanced all the time. But there is also an automatic balancer built in, see the manual for details.

Many boat owners with LFP batteries feel that they do not need any shore power at all because LFP batteries are quickly recharged by the alternator. But if you are on shore power you can use your existing charger for lead batteries. The easiest installation then is to switch the charger on and off manually, and let the LFP4S alert you with the buzzer/alarm when the battery is full and it is time to unplug the charger. It is of course possible to have the BMS4S to switch the charger on/off automatically. How is described in the user manual. You can also buy a special charger for LFP. Some of these can easily be connected to the BMS4S to let it control the charging process. Or if you have kept the "starter battery" as backup, you can use it as main battery while on shore power and keep the charger on constantly. But as I said, many people rarely need shore power, so the manual method works for most people.

Other charging sources like solar cells, wind generators etc are controlled the same way via relays connected to BMS4S.

There are many other possibilities you can use BMS4S for: duplicated system for increased safety, mono-stable relays, temperature monitoring of generator and batteries, etc. This is also described in the user manual, that you can find here.

A summary of what this solution is:

  • is made for 12 V LFP (4 cells)
  • always act on the first cell reaching a boundary
  • has several levels of handling low voltage
  • controls the alternator via the voltage sense cable, existing regulators will work fine
  • can protect the alternator from overheating
  • can control chargers not made for LFP
  • has three logging functions to make it easy to understand how the system performs
  • estimates SoC reasonably well, without need of synchronization
  • estimates level of unbalance, by coulomb counting while charging
  • can balance up to 1.5 Ah difference per day, balancing also when not charging so no long charging times are required
  • two BMS can be configured to work in parallel for highest reliability where they supervise each other
  • can directly handle bi-stable relays, but can also handle mono-stable relays
  • drives relays that requires up to 7 A momentarily and 1.2 A constantly
  • communicates over Bluetooth and via an FTDI/TTL-serial serial cable
  • has few limitations on how it can be configured

It comes pre-configured, with a profile we have concluded is a good compromise between capacity usage and battery life-length, ready to just install. But for those that have own ideas, almost everything can be reconfigured, also to profiles that we think is unwise. But if you are missing something, mail info<at>hlpdata.se and describe your problem. We can easily introduce additional working-modes with alternative usage of existing ports.

If you want to know more about LFP and why BMS4S workes as it does, you can read about it here.

And you can see how I have installed it i my boat here


This solution has now been i service for three years and is now available for DIYers...