Should I replace my batteries with lithium? Why you should (& why you shouldn’t)

Late spring/early summer it’s when the flowers start to bloom, the weather starts getting warmer and I start getting phone calls from boat owners claiming that their battery chargers are not working. 9 times out of 10 it’s not the charger but their batteries that have died from lack of charge over the winter. There is a mixture of reason that this happens, sometimes deliberate, sometimes accidental, but nearly always ends up with the need to buy new batteries. I’m often asked about lithium batteries but the price usually puts them off and they’ll go with like for like replacement of their old batteries, but lately more and more people are taking the plunge and investing in this new technology. Today’s post is to clarify the facts for you to help you make a decision for yourself if you’re in the market for new batteries either now or in the future.

It’s hard to find information on lithium batteries out there in internet land that is not biased from companies either trying to sell you their lithium batteries or other companies trying to sell you their alternatives. I have no such conflict in interest and I am happy to help whatever one decides. I’m sure for most people this is a decision based on finances, but at the same time the costs whatever the decision are not as straightforward as they seem.

You may have seen or read or been told that a high quality lead acid battery is more cost efficient than a lithium battery and this is can be true depending on your own set of circumstances, but there are many situations where quite the opposite is true. On the other side of the coin lithium battery manufacturer’s will often quote that their batteries are cheaper in the long run due to their superior cycle life, however to see if that’s true let’s go over the stats.

Note: when discussing lithium batteries i’m talking about the commercially available Lithium Ion Phosphate batteries (LifePo4)

1 Initial Cost.

In comparing the data, all the batteries are from the same dealer in Spain and with exception of the Flooded Lead Acid batteries they are all from the same manufacturer.

Actual price you pay will depend on brand, where you are in the world, quality, quantity, capacity, etc. But again for comparison purposes I’ve given a per amp hour cost based on average prices here in Spain.

CostCapacityTechnologyPrice Per Ah
214€135AhFlooded Lead Acid1,6€
289€106AhLead Carbon2,7€
1000€100AhLithium Ion Phosphate10€
Average online battery price in Spain

And the most costly are Lithium batteries at 10€ per Ah.

The cheapest at 1,6€ per Ah is the flooded deep cycle lead acid.

2. Cycle Life.

To really understand what you’re getting for your money, we next need to look at cycle life, using the per Ah cost I’ve estimated the per cycle cost by rounding each battery to 100ah and divided each battery’s estimated cycle life as per their data sheets.

Note: DOD stands for Depth of Discharge i.e. how much of the batteries capacity you use each cycle

TechnologyPer 100Ah CostCycle Life of Battery @ 50% DODPrice Per Cycle @ 50% DODCycles Life of Battery @ 80% DODPrice Per Cycle @ 80% DOD
Flooded Lead Acid160€60026cNot recommendedNot recommended
Lead Carbon270€100027c50048c
Estimated per cycle cost of a 100Ah example of each battery type.

3. Charging efficiency with shore power

We now have a picture of the costs of the different technologies based on the batteries themselves, but the story doesn’t stop there. There is also the cost of charging them which differs between the battery types. The figures below are estimated using the average Spanish shore power cost of 26c per kw hour, again the actual cost will vary as to the cost charged by your marina, but what won’t change is the cost ratios between each battery type or the rankings as per efficiency.

TechnologyEfficiencyCharging cost per 100Ah @ 50% DODCharging cost per 100Ah @ 80% DOD
Flooded Lead Acid80%16c26c
Lead Carbon95%13c22c
Per cycle charging cost from shore power after efficiencies are taken into account.

Efficiency is the amount of power that is delivered to the battery vs the amount absorbed. In the examples above if you were to deliver 100Ah to the the Flooded Lead Acid battery when completely empty, only 80Ah would be absorbed by the battery itself but you would still be charged for the full 100Ah by the marina. You would then still need an additional 25Ah to fully charge it.

For the Lead Carbon Battery and the Lithium you would be almost fully charged with 95Ah of the 100Ah ending up in the battery with only 4,75 extra Ah needed.

4. Charging efficiency with generator

Charging with a generator is where the comparison becomes interesting, small marine generators are not the most energy efficient pieces of machinery to begin with but they are even less so at low loads, in this state you are basically burning fuel for the sake of burning fuel with the side effect of a bit of electricity. As a result when charging by generator we tend to base our cost on run time as opposed to kilowatt or amp hours.

TechnologyRecommended Charge RateTime to charge from 50% DODCost to charge at 50% DODTime to charge from 80% DODCost to charge from 80% DOD
Flooded Lead Acid0,1C4hrs 10min5,40€6hrs 40min8,66€
Gel0,2C2hrs 46min2,91€5hrs 30min7,15€
AGM0,2C2hrs 55min3,79€5hrs 42min7,41€
Lead Carbon0,2C2hrs 37min3,40€4hrs 12min5,46€
Lithium0,5C1hr 3 min1,37€1hr 40 min2,16€
Per cycle charging cost by generator after efficiencies are taken into account.

The above figures are calculated using diesel costs only (1,30€ at time of writing) for comparison purposes. If we were to add engine wear and maintenance costs we could easily quadruple our figure, but also quadruple our savings using lithium batteries.

Another point to make is the above are also based on the assumption that all the above batteries would be charged using a smart charger that stops its absorption cycle when the current flow drops below a certain threshold, some chargers use a timed absorption cycle which would add even more charge time to the above charging times and therefore the cycle price

5. Calendar life.

If you use your batteries in a low cycle or standby capacity then calendar life takes over from cycle life as the major concern factor. The times quoted assume that said batteries are kept at a temperature around 25 deg C

The Lead carbon and Lithium batteries are purposefully estimated quite conservatively as they are relatively new technology and therefore cannot be verified in the same way as Flooded, Gel and AGM can. That said it’s likely that both these batteries will most likely last significantly longer however at this stage the manufacturers are only committing to the 10 year figure.

Technology Average Lifetime Annual Cost
Flooded Lead Acid10 years (if properly maintained)21,4€
Gel8 Years31,2€
AGM5 Years46€
Lead Carbon10 Years 28€
Lithium 10 Years100€
Annual cost of different battery technologies when in a stand bye setup (not including charging costs)

As you can see when used in this way there is a reversal in cost efficiency. The type of vessels using this setup would be larger boats in the 25m+ range that use permanently running generators or smaller boats with a relatively low house loads that can be totally powered by the alternators.

6. Maintenance.

For you to take advantage of the lower cost of flooded lead acid batteries in a standby set up this is the most important factor you need to have covered. Otherwise any savings are quickly lost as like some of my clientele mentioned above you end up forking out for a new set. If you have full time staff or your own time (and the will) to carry out the maintenance required then a lead acid setup can in these situations offer some overall cost saving.

The VRLA or sealed lead acid technologies (AGM, Gel etc) sometimes called Maintenance free (which can be a bit misleading as while they do take away the messy/laborious/time consuming part of maintaining a battery there is still some maintenance required in the way of keeping them fully charged). These benefits come at a cost both at an initial outlay and in battery life.

The newer Lead Carbon style batteries do offer a significant improvement over the older technologies but still require some maintaining and as expected cost even more.

Lithium batteries require almost zero maintenance (depending on brand) and in most cases are the outright winner on this factor.

7. Gassing

This factor is the flooded lead acid battery’s biggest disadvantages but at the same time one of its smaller advantages. If you have an older boat already with these batteries then its probably safe to say it has been designed to cope with this hazard (i.e. vented battery box). The sealed battery technologies don’t usually have this problem and therefore have greater flexibility as to where they can be installed. However they can still vent gas if accidentally overcharged which translates to irreversible loss of capacity and possible premature death if overcharged too long. Flooded lead acids can be resurrected by topping up the water in these instances.

It goes without saying that lithium doesn’t have this problem, although death by overcharging can still be a concern if not installed properly.

8. Safety

As I said above when we are discussing Lithium batteries in this write up, I’m talking about a type of lithium battery called Lithium Ion Phosphate (LifePo4) which is much safer than their cousins Lithium Cobalt Oxide (LCO) which are mainly used in laptops, phones, watches etc and Lithium Nickel Manganese Oxide (NMO) used in Electric Cars both which have been implicated in the overheating/catching fire stories that you have been circulating.

As per the last point lead acid technologies also have safety risk but most of these can be minimised to an appropriate level if installed properly.

9. Useable Capacity

This is another win for lithium, most of the major lithium batteries manufacturers when quoting capacity are quoting usable capacity as opposed to theoretical capacity. What I mean by that is say when a lead acid type manufacturer labels their batteries as being 100ah then unless otherwise stated you can really only make use of half of that capacity before the battery starts becoming damaged. A similar sized major brand lithium battery would usually make use of the full quoted 100ah as in reality they often have an extra reserve capacity of about a quarter (making them 125ah) that is not usually used

Lead acid technology is catching up with some manufacturers claiming their batteries can now be discharged to 80% however this comes at a cost of longevity.

10. Lighter

This factor is not even close, Lithium batteries are at least a third the weight of an AGM or Gel battery of the same capacity and less than half of a flooded battery equivalent.

Boats in the up to 25m range with large battery banks will notice a large difference on how their boat sits on the water after changing to lithium, less weight = greater fuel economy.

11. Temperature handling

This factor all depends on where your boat lives and where you plan to take it. If its likely your boat will spending a lot of time in below zero temperatures then lead acid technology might be the better option as lithium batteries don’t perform as well in these conditions (although some of the manufacturers are producing heated batteries to negate this problem)

On the other side of the coin if your boat is is spending time where the batteries are exposed to heat over 20 degrees celsius then lithium is the better choice as they are less affected by heat, as a general rule for lead acid batteries every 10 degrees over 20 deg c you are doubling its ageing effect.

12. Self discharge

The comparison of self discharge between a high quality sealed lead acid and lithium batteries are generally negligible (around 2-3%) however following on from the last point on temperature, higher temperatures also increase the self discharge rate but the effect is more pronounced with lead acid batteries.

Although with all conditions being equal, there is a fact that puts lithiums in front and that is lithiums don’t mind being in a partially charged state where as lead acids do, so much so that if left discharged for too long lead acids can die an early death.

13. Required modifications

This all depends on your existing setup vs your planned setup. Obviously if your planning on replacing your batteries like for like then there are no modifications needed (although if you are experiencing earlier than expected failures you might want to review your current setup).

If upgrading from flooded lead acid to AGM or Gel you will want to make sure that your chargers have a setting to support that technology otherwise you may be needing new batteries again sooner than anticipated.

Lithium looses out on this factor as there is a high probability that some modifications will definitely be in order. Just how much is dependent on battery type, current setup and planned setup (I will go into further detail about different setups in a future post) but the bottom line is replacing your lead acid batteries for lithiums its not simply a matter of taking your old batteries out and dropping new ones.

11. Environmentally friendliness

This is not as straightforward as it seems as there are many variables to consider but generally if we follow the full life of the batteries from production, through their working life to disposal, lithium comes out in front.

Lithiums are by no means perfect in the environmental sense, however when compared to Lead acid batteries they use a lot less raw materials and a lot less energy in their production. They use approximately 20% less energy to charge when in use which can lower your carbon footprint if your charger is connected to power sourced from fossil fuels. And they are less toxic too, if accidentally released into the environment.

Where lead acid regains some of its environmental reputation is the fact that at the time of writing this, there is already a quite sizeable infrastructure for the recycling of lead acid batteries where up to 98% of the battery can be recycled into new batteries. This simply does not exist in the same scale for lithium batteries. However as time goes on no doubt this will change.


If you own or operate a boat with a decent sized battery bank that spends a significant time with the generators running then changing to lithium is definitely a worthy consideration as the savings will quickly add up. If that same boat spends a significant part of its life plugged in at the marina then there are still savings to be had by changing to lithium batteries however those savings won’t be as high.

For larger boats with full time running generators where the only batteries you have are starter, engine control and navigation, then lithium batteries make less financial sense, however with prices coming down all the time its possible that could change in the future.

On the other end of the scale smaller recreational boats without significant house loads like fridges, inverters etc are also unlikely to benefit from lithium’s superior cycle life.

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