Voltage and battery discharge

Why are we always talking about 12-volt or 24-volt batteries, but do they never deliver exactly 12 volts or 24 volts? This is because each battery always delivers a slightly higher voltage when the battery is fully charged and a lower voltage when the battery is empty. So when we talk about a 12-volt, 24-volt or 36-volt battery, we are talking about the voltage of the devices the battery can supply power to. A 12-volt lead-acid battery that is fully charged often provides a voltage of about 12.7V. If the lead-acid battery only has 20% left, it will only deliver 11.6V. A fully charged lithium battery delivers 13.6V but delivers 12.9V at 20%. Since most trolling engines and other equipment have been designed for use with lead-acid batteries, Rebelcell developed the AV line (AV stands for Adjusted Voltage). The batteries in the AV line have a lower voltage than regular lithium batteries. This means you don’t have to worry about your engine burning out. The table below shows voltage of a lead-acid battery, regular lithium battery and a AV line lithium battery Depending on the chosen battery technology, the actual discharge curve of each battery may vary.

Capacity in % Lead-battery Lithium battery Lithium AV-battery
100% 12.70 V. 13.60 V. 12.60 V.
90% 12.50 V. 13.32 V. 12.10 V.
80% 12.42 V. 13.28 V. 11.60 V.
70% 12.32 V. 13.20 V. 11.35 V.
60% 12.20 V. 13.16 V. 11.10 V.
50% 12.06 V. 13.13 V. 10.80 V.
40% 11.90 V. 13.10 V. 10.70 V.
30% 11.75 V. 13.00 V. 10.60 V.
20% 11.58 V. 12.90 V. 10.45 V.
10% 11.31 V. 12.00 V. 10.25 V.
0% 10.50 V. 10.00 V. 9.00 V.

When you use a 12V battery with a DC stabilizer, the output is always stable at 12V. The DC stabilizer is designed for devices that do not accept an input voltage that is too high or too low. Suppose you have a device (for example a depth sounder) with an operating voltage of 10.5V-12.9V then you need to use the DC stabilizer with a lithium battery. With a “normal” lithium battery because the maximum voltage of 13.6V is above the maximum voltage of 12.9V of the depth sounder. With a battery from the AV line you need the DC stabilizer because the minimum input voltage of 10V is above the minimum voltage of 9V of the AV battery. In this case the device would switch off while there is still 25% of capacity in the battery remaining.

Effective battery capacity

The effective battery capacity can sometimes differ from the nominal battery capacity. This means that in in practice a 100Ah battery can deliver fewer Amps than the 100 Amps that are specified. This is mainly the case with lead-acid batteries. In the case of lithium batteries (such as those from Rebelcell), the effective battery capacity is very close to  the nominal battery capacity. Why is this different for lead batteries? There are two reasons:

  1. A lead battery should never be fully discharged.
  2. Battery capacity is ‘lost’ when a lead battery is discharged faster.

1. A lead battery should never be fully discharged

Lead-acid batteries can only be discharged up to 50% before irreversible damage occurs. In practice this means that you will only be able to use half of your battery capacity. Semi-traction batteries such as AGM en Gel lead-batteries are often used as Marine batteries for water sports and angling. These batteries can often be discharged up to 70%. This means that in practice 30% cannot be used. Lithium batteries can be fully discharged which means you have the full capacity available to use. In addition, unlike lead-acid batteries, Rebelcell’s lithium batteries are protected against deep discharge damage thanks to the Battery Management System (BMS). The BMS willl  “automatically” switch off the battery when it drops below 3%.

2. Battery capacity is ‘lost’ when a lead battery is discharged faster.

In the case of a lead-acid battery, the capacity is always indicated in, for example, C1, C5 or C20 (or a C with a different number). The C stands for capacity and the number stands for the number of hours in which the capacity can be delivered (C-Rate). For example, if a battery says C20=100Ah, this battery can deliver a total of 100Ah if it is discharged in 20 hours. So a device that draws 5 Amps can run for 20 hours on this battery. However, if the battery is discharged faster, the total capacity drops radically. The same battery can have a C-rate of C5=70Ah. If you disconnect the battery in 5 hours it will only deliver 70Ah in total. This is because the internal resistance of a battery increases when it is discharged faster and capacity is lost by heat. By default, lead batteries are often measured over 20 hours discharged (so C20).

The faster a lead-acid battery is discharged, the less capacity it has. While with lithium batteries this is not the case. For a Rebelcell 12V50, for example, C1=C5=C20=50Ah applies. The effective battery capacity therefore depends on how deep you can discharge a battery, and how much energy is lost due to the speed of discharge of your battery.

Example 1: You use a 12V105 Ah semi-traction AGM Marine battery to power a Minn Kota Endura Max 55LBS trolling motor. For this battery the following C-Rates apply: C20=105, C5=85, C3=70. The maximum discharge % of this battery is 75%. If you use this battery with a trolling engine the applicable C-Rate is C3=70Ah. Your effective battery capacity is therefore 52.5Ah. So in practice the runtime of a 105Ah semi-traction battery is about the same as a Rebelcell 12V50Ah lithium battery.