Emrhys Barrell tests a dozen 12V lithium batteries (60-120Ah) with integral charge monitoring and the results are surprising
The generally accepted rule of thumb with lead-acid is that your battery pack capacity should be double what you are likely to use in one day, or between recharges. This is because most lead-acid batteries should not be regularly discharged to below 50% of their capacity, or their cycle life will rapidly diminish. Some lead-acid technologies will improve on this, but at greater cost.
At the same time the faster you take power out of a lead-acid battery, the less overall capacity you will get. Typically a battery that will give you 100Ah if it is discharged at 5A, ie over a 20-hour period, will only give you 80Ah if you discharge at 20A, ie over 5 hours, and 50Ah if you discharge at 100A, ie over a nominal 1 hour. This becomes especially significant if you are powering an inverter, or driving an electric boat.
Thus your nominal lead-acid battery capacity will have to be even greater if you are regularly taking out high currents.
In addition, the number of useful cycles any battery will give you over its lifespan is governed by how low you take its charge each time, known as Depth of Discharge (DoD), and how fast you take the power out.
The cheapest lead-acid leisure battery on sale in your chandlery will probably give you at best 100-125 cycles at 50% DoD. Regularly take this down to 80% DoD and the figure will drop to 25-30 cycles, at which point you will probably only have 50-60% of the original maximum capacity remaining. If you take the capacity down to 90% DoD – when the lights start to dim – you will probably get 10-20 cycles at most.
Better leisure batteries on sale will improve on these figures, but usually not by much, and you’ll generally have to look to traction batteries, designed to power electric vehicles, to get over 500 cycles at 80% DoD. Deep Cycle batteries from Vetus, and Lead Carbon batteries from DBS Leoch both claim even greater cycle lives.
And the final bugbear with a lead-acid battery on your boat is sulphation. Leave it over winter at 80% charge, and that will become its maximum capacity, no matter how much you charge it up afterwards.
Do that two years running and you are down to 60%.
Are Lithium batteries better than Lead-acid batteries?
Lithium batteries claim to turn all these performance expectations on their head. They claim discharge capacities of nearly 100% even at 100A discharge, and cycle lives of 2000+ at similar discharge currents and 80% DoD.
The aim of our exercise was to put these claims to the test.
To ensure fairness across the range we charged each battery on test with an approved lithium charger – either the one supplied with it, or with a Victron Smart Charger, or Mastervolt ChargeMaster Plus. We then carried out three measured discharges (with recharging in between each) and measured the Ah they delivered:
1. At 25A – using an MK constant current discharge tester
2. At 100A – by connecting each to an inverter, with a 1kW electric heater plugged in
3. At 150A – as above but with a 1.5kW heater
4. Finally we measured the maximum current they would deliver for 5 minutes, on the grounds that this is what you would use to boil a kettle or power a microwave.
- Some batteries being tested only had a 90Ah or 60Ah rated capacity, so we only took 90A and 60A from these to test actual performance compared to claimed performance.
- Some batteries could deliver the 150A, some could not. In all cases it was the internal battery management system (BMS) that determined when they shut down, with different manufacturers choosing different cut-off points that they thought would enable their unit to deliver the best long-term results and cycle life.
- The temperature of our test facility varied between 10-15°C. Most makers’ stated capacities were achieved at 25°C, so our results were bound to be slightly lower than the claimed figures, by approximately 2-4Ah.
Lead acid vs Lithium batteries: the facts
For over 100 years most rechargeable batteries used lead and sulphuric acid, first invented in 1859 by Gaston Plante. Then in 1970 along came lithium batteries which are 50% lighter, with no memory effect, up to ten times the cycle life, and able to give nearly all their rated capacity at even the highest discharge rates.
But lithium does have downsides. The first is cost, at up to four times the price of an equivalent output lead acid battery, and the second is safety. Lithium is a highly reactive metal that reacts violently with water, or even moisture in the atmosphere if the cell is damaged, and if it catches fire, using water to extinguish it only makes the situation worse.
Using compounds of lithium significantly reduces, but does not eliminate the fire hazard. Lithium cobalt oxide (LiCoO2) has high energy density but greater safety risks especially when damaged. Lower energy but safer compounds include lithium manganese oxide (LiMn2O4) and lithium iron phosphate (LiFePO4). The latter has become the material of choice for moderate performance batteries, and accounts for all the units we tested.
What is in a Lithium battery?
The actual battery consists of a multiple pack of basic cells, and these can vary considerably in size, and shape. They can be flat in format, known as prismatic, in which case the battery may only have four in total. Or they can be cylindrical, in a ‘swiss-roll’ format, with strips of anode and cathode rolled together with a separator in between.
These cylindrical cells can be remarkably small, around the size of an AA battery for the 18650 size, with the result that an 85kW pack, as used in the Tesla car, has no less than 7,104 of these cells!
The nominal voltage of the lithium-ion cell is 3.2V, which means that multiples of four of these cells give you a battery with a nominal voltage of 12.8V, which closely compares to the lead acid battery, which has six cells of 2.1V and a voltage of 12.6V. This allows you to make a straight swap of a lithium battery for lead-acid.
Lithium batteries and BMS (Battery Management Systems)
In order to preserve the expected life of a lithium battery, and to maintain safety, it is essential that it is not discharged below a certain voltage, at a maximum current, and the cells are kept at equal voltages. Also the charge current and voltages should not exceed certain values. For large battery packs in cars this has required a separate and sophisticated electronic battery management system as well as a purpose-designed battery charger.
The breakthrough with the current new crop of marine lithium batteries is that they all include integral battery management systems, either in the case or as a simple plug-in device, and as a result of this they can be charged with existing chargers.
The only stipulation is that the charger must deliver a maximum voltage of 14.4V, typically a gel lead-acid setting.
However some of the suppliers indicated that if you are going to get the maximum capacity into their batteries, you should use a charger with a specific lithium setting, and some supplied us with one.
Or if the battery is in a boat or motorhome, with an engine-driven alternator, they recommend you fit a battery-to-battery charger between the engine start battery and the lithium service battery, to give maximum charge rate, while protecting both the alternator and battery from excessive currents.
The Battery Management System (BMS) is the key component of all lithium batteries and battery packs. If the voltage drops below a certain preset level, usually somewhere between 9.0V and 10.0V, or the current goes too high, the BMS switches off an internal relay and the current stops.
If this happens, some manufacturers leave the battery permanently switched off until a charge voltage is applied to it, whereas others allow the battery to reset after a few seconds once the load has been disconnected.
The BMS will also balance the individual cells in the battery – that is bring all the voltages to the same level. This is important for long life. It will also shut the voltage across the terminals down for transportation.
Lithium batteries for engine starting
None of the lithium batteries we tested claim to be suitable for engine starting. For this you need a very high current for short bursts – in the order of 200-400A, and they are just not designed to produce this regularly.
Lithium battery warranties
The warranty figures quoted for lithium batteries should be treated with caution. Most are based on figures provided by the makers of the cells, and these can often be hedged with caveats, such as tapering replacement values, rather than full replacement, and stipulations about how the battery has been charged and discharged, which could be difficult to prove if you have a claim.
For this reason you should read the small print carefully, and not assume that a unit with twice the warranty period is necessarily twice as good.
Lithium battery lifecycle claims
We could not verify the cycle life claims of the batteries we tested, but they are all between 2,500-5,000 at 80% DoD (depth of discharge), so are unlikely to all be wrong.
Having said that, when we asked the suppliers how they arrived at these cycle life claims, most admitted that the actual battery had not been cycled 5,000 times by them – it would take around 10 years to carry this out – but were based on figures quoted by the manufacturers of the individual cells inside the batteries.
And hereby comes one of the possible issues with lithium batteries. Nearly all lithium cell production is taking place in the Far East, and production standards and safety standards in their manufacture are difficult to verify. While there is an international standard for Transportation (UN38.3), as far as we can find there are few – if any –internationally agreed standards for battery production.
This is an issue that is concerning the UK Government Office for Product Safety and Standards. PBO spoke to them during our test, and they confirmed they are looking into the question of all batteries, including lead-acid, but in particular lithium, with regards to safety in use, and claimed performance, and safety when being transported.
They are looking at composition, capacity, labelling and chemistry, and their first report is due out soon. We will keep you updated on their conclusions, but you can check for yourself at: gov.uk/guidance/batteries
This is not to say that the batteries we have tested are unsafe, but we would suggest that you ask the supplier of any battery you are thinking of buying to supply you with any relevant test reports supporting compliance and safety.