Li-on Battery simulation

[Eminthepooh]

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 as well as the capacity vs discharge rate table for 5 discharge rates.

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

Thanks

 

[IRstuff]

Volts?

TTFN

FAQ731-376

 

[itsmoked]

Amp-hours times the battery's nominal voltage gives the Watthours available - with lots of gotchas.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Eminthepooh]

Let me explain a little more...

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.

 

[itsmoked]

Groan..
With Lead Acid you can essentially equate the available capacity by knowing the terminal voltage.

With LiPo you have to do coulomb counting. That means you essentially need an amp-hour meter in the battery leads. You have to actually count the electrons going and coming from the battery. You'd also need to know what the full capacity of the bank is (today) to allow the bookkeeping to be carried out successfully.

I don't see a battery; ammeter or amp-hour meter in your list.

Otherwise you may be able to see the LiPo voltage crashing each night and can make some inference from that about you capacity...

Are you sure your batteries are in parallel?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Qmarcin]

If you are working with lithium-ion batteries and your operating voltage is 8,2-6,2 so my guess is that there are 3cells in parallel and two in series (3P2S).
Regular lithium-ion battery has a OCV vs SOC characteristics well defined (opposite to ferrophosphate lithium ion battery which has a flat discharge voltage characteristics).

Thus if your all devices have constant power characteristics the battery will have to compensate voltage decrease with higher current ratio.

Anyway, basically there are two methods of calculating SoC - charge counting (Coulomb counting) and look-up table from OCV vs SOC.
For the first one there are several factors that must be taken into account because 1:1 dependency is possible only in nominal conditions (thus higher currents, elevated or low temperature, internal resistance changes over the time can alter the results of SoC calculations accuracy).
And you need extra part in your system - current sensor.

The second - voltage based by the definition tells you, what is the condition of the SoC estimation to be correct. Battery voltage must be equal to Open Circuit Voltage - a potential when there is 0 current flow between electrodes.
For systems that are working all the time you see voltage far from OCV due to current effect (drop under discharge or rise during charging) that is also dependent on the temperature, internal resistance and plenty of other parameters.

So, not going into details, dependently on your equipment and required SoC accuracy you can go into current measured based SoC that will have to integrate correction factors for temperature and current rate or voltage based where you can make mathematical compensation for voltage drop but you need to get a good SoC vs OCV vs Ri vs T characteristics from battery manufacturer.

For getting from Wh to Ah you just divide nominal capacity by nominal voltage and you get nominal capacity in Wh but keep in mind the voltage not being equal on all SoC levels.