# How Much Battery Capacity Do I Need?

The battery capacity you will need for your solar powered ham radio station depends upon several factors (I’m also assuming that you will be using a Sealed Lead Acid (SLA) battery.)

- The radio’s current draw on receive
- Percentage of the time you expect to be in receive mode
- The radio’s current draw on transmit
- Percentage of the time you expect to be in transmit mode
- How long you intend to operate

Let’s say:

- 2Ah on receive
- 20Ah on transmit
- 50% receive / 50% transmit
- operating 4 hours per day

Since I’m in receive mode half the time, and consume 2Ah on receive, that means I’ll consume 1 amp in receive mode per hour.

Since I’m in transmit half the time, and consume 20Ah on transmit, that means I’ll consume 10 amps in transmit mode per hour.

My hourly current draw is the sum of those two, or 11 amps.

If I’m going to operate for 4 hours per day then I need 44Ah of capacity.

Expressed as a formula, it looks like this:

((Pr * Ir) + (Pt * It)) * Ho

Pr = Percentage of an hour in receive mode, expressed as a decimal from 0 to 1

Ir = Current draw in receive mode, in amps

Pt = Percentage of an hour in transmit mode, expressed as a decimal from 0 to 1

It = Current draw in transmit mode, in amps

Ho = number of hours of operation

Using our example of 44Ah of needed capacity, let look at batteries. The following chart is from solarnavigator.net:

Battery State of Charge | Battery Voltage |

100% | 12.7 |

90% | 12.5 |

80% | 12.42 |

70% | 12.32 |

60% | 12.20 |

50% | 12.06 |

40% | 11.9 |

30% | 11.75 |

20% | 11.58 |

10% | 11.31 |

0% | 10.5 |

What this chart says is that if your battery’s voltage is 12.2V then it contains 60% of its rated capacity. It’s important to note however that if you use up 100% of the battery’s rated capacity you have killed it, meaning you have drastically shortened its life. In fact, to preserve battery life you never want to get into the yellow zone in the above chart. This means that if it says 20Ah on the side of the battery it really means that you can use only 14.4Ah (the top 60%) without permanently damaging it.

To be technically precise, this measurement should be taken when the battery is not under load and has been resting for 3 hours, but the chart is close enough for our purposes. Also keep in mind that the industry standard for battery ratings is to assume that the discharge takes place continuously over a period of 20 hours. Longer or shorter

Using our example and the above chart, to obtain 44Ah of capacity we need to buy a battery with a capacity of 44 / .6 = 73.33Ah.

Here is the formula to account for this:

(((Pr * Ir) + (Pt * It)) * Ho) / .6

If your soar panel is simultaneously charging the battery (and if you only operate on sunny days) then you can get by with a smaller battery. If the solar panel and charge controller can deliver 60 watts (12V * 5 amps) to the battery, then for each hour of operation you’ll be recharging the battery by 5 amps. In four hours that’s 20 amps. Theoretically then you could get by with a 53.33Ah battery. Given system inefficiencies however that is probably optimistic. (I’ll update this post after I run some actual real-world tests.

And one final consideration: Batteries sitting on the shelf will gradually lose capacity over time. Trickle chargers are inexpensive and the battery should be kept on one when not in use.