Changing from Lead-Acid AGM to Lithium batteries

David Straton and Roger Halliwell.

'Nimrod', a Seawind 1160 catamaran, has had three 200 amp-hour 12 volt AGM deep-cycle batteries.

On our last cruise, I noticed that they were losing charge more quickly than usual, and taking longer to recharge. She has a Fischer Panda diesel genset, and we have hardly needed to use it since we bought Nimrod. But on this trip, we needed it quite a lot, often turning it on as we went ashore for a walk.

On our return I started to investigate the problem. I took the batteries into Battery World, who offer a free battery testing service. Two of the three batteries failed to hold their charge, and were declared dead. The third managed 95ah, not the 200ah it should have. The batteries are 3½ years old.

This has triggered a major project on how best to replace them. I thought I would share this with other boat-owners.


Lead-Acid batteries are seriously heavy, not a good thing on a catamaran. Our 200ah ones weigh 64kg each. Total weight 192kg.

The chiropractor's friend

Usable capacity

Most car batteries are Lead-Acid. Starter batteries are designed to deliver a lot of power very quickly, and then get recharged. Deep-cycle batteries are designed to be usable more slowly with more bulk power, as needed for things like fridges, auto-pilot, radar, lighting etc. Golf-carts use deep-cycle batteries, and many cruising boats use golf-cart batteries. A sealed version of these are called AGM (Absorbed Glass Mat).

Although a 200 amp-hour battery can provide 200 amps of current for one hour, or 10 amps for 20 hours, in practice it damages the battery if you let it drop below 50%. Safer to stay above 60%. It is hard to charge cheap Lead-Acid batteries above 80% from the alternators, So, in effect, you have to operate between 60% and 80% of a full-charge. Higher quality ones like Lifeline can get up to 100%, so their useful range is 60 -100%.

Lithium batteries can be drained down to 30% and charged up to 100% so their usable capacity is 70%.

Life Expectancy

AGM batteries have a life expectancy of about 1500 cycles. Lithium claims 5000 cycles @ 70% depth of discharge (DOD).

When I looked into it, lots of people said 'Great idea, but aren't lithium batteries much more expensive?'


Capacity ah
Usable range
Usable ah
Weight kg
Lifeline 12v AGM
Lifeline 2v AGM
Fullriver AGM
EV Lithium 12v
Winston Lithium

Mmmm... Interesting!

So to check out the risks.


Lithium batteries come in different types. The type used in electric vehicles are based on Lithium iron phosphate (LiFePO4) chemistry, which is safer than some of the alternatives. It seems that the critical issue is to ensure that you do not expose them to charging voltages above their safe limit. The specs on the Winston WB-LYP200AHA battery put the safe range as 4.0 - 2.8 volts in a 3.2 volt cell. With four cells in series the official range would be 16.0 - 11.2v. With a safety margin, I decided to try and stay between 15 - 12 volts. Would my power inputs be safely below 15 volts?


Power inputs on Nimrod.

1. Shore power.
2. Yanmar alternators.
3. Fischer Panda genset.
4. Solar panels.

1) Shore power comes through a Victron Phoenix Multi-plus 12/3000/120

There are various voltage output options, controlled by dip switches. See the manual page 33.

2) Yanmar alternators. Nimrod has two Yanmar 3YM30 diesel engines with 80 amp alternators.

The installation manual, on page 13, gives information about the alternators output voltages.

The Hitachi voltage regulator TR1Z-63 has a set-point voltage of 14.2 volts.

3) Fischer Panda PMS 4000 SSC diesel generator produces 4.5 kVA of 230v AC output, which is transformed by the Victron Phoenix Multi-plus 12/3000/120 discussed above. Net output 14.7 volts DC, depending on dip switch settings.

4) Solar Panels.
Nimrod has two BP 3125 panels which put out 125 watts each. They are potentially the main problem with lithium batteries, as they can produce high voltages, above the 15 volt maximum limit.

It is thus very important that the solar power sends its output to a voltage controller. Nimrod has a Steca Solarix PRS 2020.

The instructions list the output voltages as being between 13.9 and 14.7 volts.

So. All the inputs are safe.


Winston specifies a minimum of 2.8 volts per cell. I have four cells in series, so that puts the minimum limit at 11.2 volts.

Jaycar sells a Low Voltage Battery Isolator, which is attached to the positive output terminal of the battery. If the voltage drops below 11.9 volts, it cuts the current.

I have set the alarm on the BEP Matrix Battery Monitor to go off at 12 volts, so hopefully I will get a bit of warning before all the house electrics go off. But this device should protect the lithium batteries from being damaged by undercharging.


I went with the quote from EV Works, a company in Perth. Their bloke Tim Brunner was very helpful in talking me through the issues. I bought eight 3.2 volt 200ah cells, with the idea of linking them to create two banks of four cells in series, making 12.8 volts, in parallel, making 400ah.

About a week after purchasing them online, they arrived neatly crated up.

I checked their voltages.

Four cells of 200ah in series exactly correlates with the old 200ah 12 volt battery. The four together weighed 32 kg, just half of the weight of the Lead-acid AGM.

Then followed some thinking, aided by Lego, about the best way to connect them all up. Should I replicate the wiring of the two AGM batteries by creating two banks of 12 volts?

or would it be better to march them onto the Ark two by two, in parallel, and then link all the pairs up in series later?

I decided to go with the first option, partly because I didn't have enough connectors for the second option. It also made it easier to copy the original wiring.


So much easier to carry eight loads of 8 kg onto a boat, than to carry three loads of 64 kg off it!

The new batteries have M12 bolt connections, so the pre-existing leads all needed new terminals crimped on.

Using a diagram on the iPad to avoid making any mistakes.

See the Low Voltage Battery Isolator on the right.

The settings of my BEP Matrix Battery Monitor needed some changes. In particular Lithium batteries have a much higher charge efficiency of 99%, compared to AGM batteries with about 85%.

The Peukert Exponent is a number that reflects the fact that a battery supplying a big current has less capacity than one supplying a small current. Its a problem with lead-acid batteries, but less so with lithium. So the number in the battery monitor needs to change from about 1.25 down to 1.1 or 1.05.

Peukert's Law is demonstrated by pouring beer into glasses! If you pour it in quickly, only a small amount of liquid is transferred: the rest is foam. The slower you pour the beer, the more beer and the less foam enters the glass.

You may need to test this a few times!

If you read about Lithium batteries on the internet, you will come across strong advice about the need for a Battery Management System. I have been advised by EV Works that with a small system like this, with only four cells in series, that it should not be necessary, beyond the Low Voltage Battery Isolator.

For owners of smaller yachts and monohulls, the above systems can be scaled down to suit. Roger Halliwell needed new batteries in his Noelex 30 and replaced two 100Ah Lifeline batteries with two 90Ah 12 volt lithium batteries from EVWorks. The cost was similar to new Lifelines, with the benefits of a weight reduction from 60kg to 28kg, functional capacity increased by 35%, and we would expect far longer life expectancy, up to 2,500 cycles which in practical terms is for ever. These are charged by solar, alternator or 240 charger set to the ideal voltages for lithium and perform to expectation. This setup has been operating well for several months now with no problems.


The Lithium batteries have replaced the AGM batteries with the following gains.

  1. Weight reduction from 192kg to 64kg.
  2. Usable capacity increase from 120ah to 280ah.
  3. Space saving by nearly half.
  4. Life expectancy increased by about three times.
  5. Money saving of $500 to $1,200.

Useful FAQ

Many thanks for assistance from:

* Graeme Nolan, from Seawind.
* Tim Brunner, from EV Works