1. Guys,
I'm trying to model a satellite battery life in excel. (lion polymer batteries)
I know how much power i'm getting from solar panels, and I know how much power and current I'm draining on a minute to minute basis.
I have a 3.75 Ah battery and have their Voltage vs. Capacity curve charts and stuff.

Anyone know how to go from Watts to Ah? I'm having trouble applying it to a minute by minute basis

help?

2.

3. 1. Is this homework?

2. What have you figured out so far?

4. Originally Posted by eminthepooh
Anyone know how to go from Watts to Ah? I'm having trouble applying it to a minute by minute basis

help?
Watts is a measure of power and amp-hours is a measure of battery capacity, so you cannot convert one to the other. If you know the voltage of the battery, then that number multiplied by the current is the power.

5. It's not Homework, It's for my Job.

Basically, we have three 1.25Ah batteries in parallel with a voltage of between 8.2 and 6.2 V.
I'm sourcing some devices from the batteries as well as feeding the batteries power thru solar panels.
I have manufacture literature showing me Capacity vs Voltage. I also have tables showing the capacity as discharge rates change (ie C, C/2, C/10)

I know how much power the solar panels receive during each minute the satellite is in orbit.
I know how much power and current i'm draining on a minute by minute basis.
I need a way to track and predict the Battery State of Charge.

6. Disclaimer: I am not an expert on batteries.

Here is how I'd do it, which is probably not totally accurate, but I think it would get you somewhere in the ballpark.

You are starting with a fully charged battery which is 3.75 amp-hours. At the end of a minute you have 3.75 amp-hours minus the number of amp-hours consumed during that minute.

Now we figure the amp-hours consumed during the minute. If we are discharging the battery at the optimum rate, the amp-hours consumed will be discharge rate in amps multiplied by 1/60th of an hour, where the discharge rate is the difference between the load current and the charging current. The charging current is the power rating of your solar cells divided by the average voltage during the minute.

If the discharge rate is something greater than the optimum rate, then I would apply a fudge factor, which would be the inverse of the capacity at the particular rate - i.e., if the capacity was C/2 at that discharge rate, then multiply the amp-hours consumed by 2.

At each successive minute, we start with the remaining amp-hours at the end of the previous minute and increment it accordingly.

The remaining charge at the end of an iteration cannot be greater than 3.75 amp-hours, so if you calculate a number higher than that, just set it to 3.75.

7. Hmm.... Aerospace. You can't simply drive out there and service it in the field if the calculations/models are incorrect, so it's important to know all about batteries.

There's an inefficiency with batteries called Peukert's Law, and it's based on the current draw. I'm more familiar with Peukert Effect with lead-acid batteries, but lithium-ion batteries also suffer from this to a lesser degree.

Basically, the more current drawn from the battery, the less efficiently it works. If your battery has, say, a 100 A-h capacity, it might output 1 Amp for 100 hours, but it'll output 10 Amps for less than 10 hours (~9 hours), and it'll output 100 Amps for much less than 1 hour (maybe 45 minutes).

Also keep in mind the voltage discharge characteristics of batteries. For example, a regular 1½-volt battery may have 1.5 volts initially, but as it continues to discharge, the voltage soon drops to about 1.2 volts and remains there for most of its useful life. So, if your load requires a certain amount of power, it will either draw more than the expected current to compensate for the lower voltage, or it may malfunction due to the low voltage. Higher currents also shortens battery life.

You also have the effects of temperature on battery performance, and it may also be significant that the temperature cycles up and down as the satellite orbits the Earth, sometimes baking in full sunlight, other times freezing in the shadow of the Earth. Chances are good that the battery manufacturer doesn't have specs on performance during temperature swings between ±100°C.

Powered missile flight also complicates the matter by subjecting the batteries to g forces and vibrations. You may find that the battery must be oriented in a particular way to avoid adverse effects from these conditions.

You might need to work closely with the battery manufacturer and with the makers of the missile.

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