It’s time. Your RV or boat’s lead acid battery bank had a good run, but just isn’t able to hold a charge anymore – so what should you do? Using battery desulphators could help squeeze some more life out of it, but instead of asking how to restore lead acid batteries that are clearly past their prime, the question you should be asking is: Can I replace lead acid batteries with lithium batteries in my boat or RV? After all, lithium batteries are becoming the standard for renewable energy storage.
The answer is YES, you can absolutely replace lead acid batteries with lithium in marine and RV applications – but here are a few considerations to help you decide if upgrading to lithium batteries is the right lead acid battery alternative for your boat, camper, or RV.
MARINE BATTERY
Depth of Discharge, or DoD, is how much of your battery bank’s stored energy can actually be used without dramatically reducing its life. For example, a 100Ah (amp hour) lead acid battery rated for 25% DoD means you need to plan to use only ¼ of its rated capacity (so 25Ah), leaving the other ¾ in the battery, unused.
● DoD for lead acid batteries – both flooded (which you have to add water to periodically) and sealed (“maintenance-free”) – is typically in the 25% – 50% range. Your battery will last at least twice as long if you regularly discharge it 25% than if you regularly discharge it 50%. Keep in mind that if you don’t have a sunny day to recharge your batteries after a day of use, the DoD will go down again the next day – so planning to use 25% per day will allow you to use less than the 50% maximum after two days of use.
● On the other hand, DoD for lithium ion batteries is 80% or more, allowing you to use most or even all of the battery’s stored energy. That means a 100Ah lithium battery rated for 80% DoD can safely provide you with 80Ah without being harmed.
As a result, a lithium battery bank can be much smaller than a lead acid battery bank to provide the same amount of usable energy. For example, if you need 100Ah of energy a day, you would need a 400Ah lead acid battery bank to stay at 25% DoD, but would only need 125Ah of lithium at 80% DoD. That is a significantly smaller battery bank with lithium batteries.
Cycling a battery means discharging it to any amount and recharging it to a fully charged state. If you cycle your battery bank every day for a year, that’s 365 cycles. If you only use it on the weekends, and keep the bank topped off the rest of the time, that’s 104 cycles a year.
A cycle is a cycle regardless of how deep the discharge is, but the depth of discharge directly affects how many cycles you can expect your battery to last. A battery’s specs will tell you how many cycles to expect from it when discharging to its rated DoD.
● A standard flooded lead acid battery can have about 2500 cycles at 25% DoD
● A standard sealed lead acid battery can have about 1200 cycles at 25% DoD
● Unlike lead acid, lithium batteries don’t have a cycle curve under 80% DoD. Beyond 80%, the cycle count can drop dramatically. A typical lithium battery can have 5000+ cycles at up to 80% DoD. That’s 4x the cycles at over 3x the DoD. That’s a much longer lived battery bank with lithium batteries.
In addition to how much of a battery’s capacity you use, it also matters how fast you use it. Again using the 100Ah battery example, if you have a 10 amp (A) load, that can drain the battery completely in 10 hours (100Ah ÷ 10A = 10 hours). That is considered a C/10 rate. Likewise, if you have a 50A load on the same battery, that would drain it in 2 hours (100Ah ÷ 50A = 2 hours). That is a C/2 rate. Most batteries are rated at their C/20 rate, emptying the battery in 20 hours.
If you have a high-current load in your system, or are charging it very quickly with a high current, such as your alternator or shore power, you need to consider the charge/discharge rate of the battery bank. If you need a higher rate than the batteries can handle, you would need to increase the battery bank by adding more batteries in parallel so that the batteries can share the current between themselves. This may result in needing a battery bank that has a higher Ah capacity than you need to power your loads, just to handle the high current.
Likewise, too slow of a charge of lead acid batteries can cause premature sulphation, shortening their life. This is not a problem with lithium.
● Lead acid batteries tend to perform best between C/8 and C/12 rates. So our 100Ah battery would want to be charged or discharged at between 8A and 12A. Wiring three batteries in parallel would permit three times the rate, as it shares the current between the three, so 24A to 36A.
● Some lithium batteries can generally handle a C/1 rate, or even higher for short periods depending on the battery. This means a 100Ah lithium battery can handle 100A (or more) of charge/discharge current. Most manufacturers recommend no more than a C/2 rate on a regular basis for best battery life, but it is good to know the extra power is there with lithium batteries if you need it. Be sure to check the manufacturer’s specs when selecting a lithium battery, as some do not support as high of a current as others.
You may be familiar with the voltage of your boat or RV’s battery bank sagging, or dropping to 11V or lower when trying to run a high-power load such as your winch, windlass, or air conditioner. When running a heavy AC load off the inverter, the voltage could drop below the low voltage cutoff, causing the inverter to turn off when you need it most. Likewise, if you are running a DC load like your bow thruster directly off the battery bank, you need it to maintain a high enough voltage for it to work when you really need it to work. Due to lithium batteries’ voltage curve and ability to handle high current, loads like these will not cause the voltage to drop dramatically, eliminating the problem of voltage sag.
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