Why are 51.2V batteries sold as 48V?

Thu Nov 30

Written by: Casey O.

One of the most confusing aspects of shopping for LiFePO4 batteries is how companies choose to market the voltages of their batteries.

On many LiFePO4 battery manufacturers websites, you will see an even-number voltage in the products title and description: 12V, 24V, 48V, etc.

However, if you look at the specs for those batteries, you might see a different voltage called the ‘nominal voltage’. 12.8V for the 12V battery and 51.2V for the 48V battery, for example. Maybe just rounding down a bit for the 12V battery, but what’s going on with that “48V” battery?

First we need to understand that in both cases, these descriptions are just approximations. The ‘nominal voltage’ of a battery is a specification or approximation of its normal operating voltage. It’s ‘nominal’ as in ‘name’, to be used for identification. It’s a convenient designation. If you measure that “12V” battery with a multimeter, it won’t always read 12V exactly. You could measure anywhere from 10V to 14.6V, depending on the charge level and whether the battery is being charged or not, among other factors. 12.8V is a convenient description of the batteries specs.

So where does 12.8V come from? Well, raw LiFePO4 cells have a nominal voltage of 3.2V. If you put four of them in series (4S), they add up to 12.8V. Okay, that’s consistent.

For decades, we have had 12V systems: car electronics, RVs, off-grid appliances and trolling motors. LiFePO4 12.8V batteries are often drop-in replacements in those systems, so using the same name to identify them is convenient and helpful. This 12V or 24V or 48V ‘system voltage’ is also sometimes described as the ‘nominal voltage’, not to be confused with the 12.8V nominal voltage of the battery that is compatible with the nominal 12V system.

Here is where it gets really, really confusing. Let’s say you’re looking for a “48V” system which are common in big off-grid and grid-tie solar setups. If we quadrupled the number of cells that we had in the “12V” battery, to have 16 cells in series (16S), we would get 51.2V, which we often see on the spec sheets of “48V” batteries. That checks out.

But what if we only had 15 LiFePO4 cells in series (15S), that would be… exactly 48V.

And some manufacturers sell batteries with 15 cells in series as 48V, which in some ways a more accurate description of the product, yet in conflict with the other convention of rounding the nominal voltage to be divisible by 12.

It’s very confusing, and I haven’t found a name for these ‘marketing voltages’ that manufacturers use when they decide to put “48V” in the product name of a battery with a nominal voltage of 51.2V. We need to be able to talk about the two different voltage values that are provided, so here are some ideas:

• ‘Marketing voltage'
• 'System voltage'
• 'Rounded nominal voltage’

I like ‘marketing voltage’ personally. It’s worth talking about, because let’s say your system could function with a 15S battery that has an actual nominal voltage of 48V, or a 16S battery with a nominal voltage of 51.2V, and they cost the same. If you chose the 15S 48V system in this case, you’d get ~6% less energy storage for your money.

Every manufacturer uses their own preferred marketing terminology, so when in doubt, read the spec sheets.