Oct 14, 2020  •  4 min

Consider a lithium battery? Measure its capacity correctly.

This blog is for lead-acid battery users who are considering the lithium battery. At the boat shows, some buyers will ask “how many ampere-hours (Ah) of this ePropulsion battery?” Normally, they ask about the ampere-hours because they have the experience with the lead-acid battery, which is always measured in Ah. They want to compare the runtime, weight, and cost between the lead-acid battery and ePropulsion lithium battery at a similar capacity by Ah. So they figure out if the ePropulsion battery is a good deal for them.

Unfortunately, they will get the wrong conclusion. First, the lead-acid battery is usually delivered in 12V, sometimes 24V, while ePropulsion lithium batteries are 48V. You need four 12V lead-acid batteries in serial connection to make a 48V battery pack. You can never compare the performance and cost between a 12V battery and a 48V battery, even though they have the same capacity by Ah.

More importantly, lithium battery has a larger usable capacity than the lead-acid battery. Normally, the usable capacity of the lead-acid battery is around 50% at 1C discharge, while lithium battery’s usable capacity can be over 97%. That’s a huge difference. To power Navy 3.0 for two hours at full throttle, it requires 6 kWh lithium battery, which is 130 Ah at 46.8 V. If you use the lead-acid battery, will a 130Ah 48V lead-acid battery power Navy 3.0 for two hours at full throttle? Definitely no, you probably get even less than one-hour runtime at full throttle. No one can’t tell you the accurate runtime because it varies heavily at different C rates (how fast a battery is being discharged) and battery brands. The lead-acid battery is just dumb and hard to estimate the accurate runtime. Please note that we’re talking about power battery for propulsion motors and you go at full throttle, not for small electronic devices like mobile phones or fish finders that consume small power.

Why does 130AH lead-acid battery deliver less runtime for Navy 3.0 than the lithium battery? The lead-acid battery has a Peukert Effect. The faster a battery is discharged, the lower the AH capacity. Conversely, if it is drained slower, say a fish finder, the AH capacity is higher. This is important because many manufacturers have chosen to rate their batteries at the 100-hour rate – which makes them look a lot better than they really are. You may refer to the useable capacity chart of Trojan Motive OverDrive AGM 31™ 100 AH below.

Let me help you to understand the chart. When the continuous discharge current increases from 5 A to 20 A (faster to discharge), the useable capacity drops from 102Ah to 84Ah. You should know Navy 3.0 requires continuous discharge current at 62.5 A at full throttle, way more than 20A, which means the battery discharges very fast. If you have four of this Trojan 12V 100 AH batteries in serial connection and you go full throttle often, you get usable capacity much less than 84 Ah.

Let’s go deeper with a voltage-capacity curve below. It shows how the voltage drops at 1C discharge. At 50% capacity, the voltage of the lead-acid battery drops to under 10V that it’s too low to power your electric outboard. The LFP battery still remains high voltage after 97% capacity consumed. This explains why the lead-acid battery has a small usable capacity for electric outboard. The voltage drops too fast when it discharges fast.

*The graph is for reference only. We assume the C-rate is 1C. It doesn’t represent the actual performance of any specific battery models. The actual voltage drop and usable capacity vary a little bit between different brands and models.

To learn the usable capacity is smaller than labeled capacity, here’s a YouTube test video (VMAX SLR100 test starts from 7:05). You will see VMAX SLR100 of 1200 Wh labeled capacity turns out a useable capacity of 782 Wh only.

In conclusion, it makes no sense to compare the performance, weight, and cost between the lead-acid battery and the lithium battery at a similar capacity by Ah. You think similar labeled capacity by Ah delivers a similar runtime, but it’s not. Again, this conclusion applies only to the power battery used for electric outboards, not small electronics.

To measure the capacity of lithium batteries, we highly recommend you use Watt Hour (Wh). The benefit is that you could calculate the runtime easily and accurately without considering the voltage at all. You divide the capacity with the power, and you will get the runtime, very simple. For example, Spirit 1.0 Plus battery is 1276 Wh, and the full-throttle power is 1000 W. The runtime is 1.276 hours (1276 Wh / 1000 W), which is 1 hour 15 min.

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