Lithium Phosphate battery performance

[Stephen White]

I have a Tracer 24v 20AH Lithium Phosphate battery, with BMS and condition gauge, in my Cent. The discharge curve for LiFePO3 is much flatter than for lead acid, ie the battery sustains its discharge voltage for longer, then drops rapidly. In the case of my battery, the voltage at the end of discharge is 16v, nominal voltage is 25.6v and at full charge its about 26.4v.

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The battery condition meter shows as follows:

- 3 green + 2 red: fully charged 26.4v
- 2 green + 2 red: Over 50% capacity 26.0v
- 1 green + 2 red: Over 20% capacity 25.8v
- 2 red: Less than 10% capacity 24.0v
- 1 red: Fully discharged 16.0v

At Model Expo on Saturday, I ran the tank several times during the day and it had a long run through the Museum, including the long ramp to the main entrance. On loading it, the battery meter showed 1 green + 2 red, voltage 25.3v. OK so far but there was a nagging worry about running out of puff before being able to load... however.....

An hour later, at home, the gauge was showing 2 green + 2 red, voltage 25.8v ie after a rest, the capacity appeared to have recovered somewhat. It's the first time I've been able to measure this performance. If LiFePO3 batteries do recover some capacity when rested after a long discharge period, it might be useful to know for those moments of desperation when you think the tank is dead and there are no charging facilities to hand. Am I imagining this or is it well known?

Stephen

[Gerhard Michel]

Hello Stephen,

though LiFes have a 'harder' discharge curve, they also recover the shown voltage a little when resting. Unfortunately this is not a true recovery of capacity, but only of 'idle' voltage!

[Christoffer Ahlfors]

It's PO4 as in phosphate, capital A for amps and common h for hours. After all, we are detail nerds here!

One dreaded property of lead/acid batteries is that their internal resistance increases rather substatially with discharge. This means that the voltage drops dramatically with high currents - far worse than the idealized discharge curves from the manufacturers, which assume a steady, modest current. I can measure 24V when idling with the smoker running (at,say, 2A), but this can then drop to below 20V when turning, making running difficult. Time for a pit stop at maybe 75% remaining capacity (as measured by the charger). How are LiFePO4 batteries in this respect?

A second question: A business case for LiFePO4 depends heavily on the life expectancy. Idealized manufacturer claims have not impressed me when it comes to lead/acid and I would not believe them blindly for LiFePO4 either, without actual user stories. Does anyone have any actual experience here?

Thanks,
/Chris

[Jeffrey Goff]

Hi Chris, I bought a liFePO4 24v 20ah battery from Ping in China about 5 years ago, and fitted it in a tiger 1, the model only gets light use once or twice a year, but the battery keeps its charge, it has only been charged two or three times over this time, whereas the lead acid without use would be useless and thrown away.
Regards Jeff

[Stephen White]

Well, Christopher, on Saturday I ran the battery down to 20% and at rest it was delivering more or less the nominal voltage of over 25v. For this particular battery, the max continuous current draw is 30 A and the max peak is 60 A.

As for the business case, I posted some figures here a while ago, which showed that although the acquisition price is higher than lead acid, LiFePO4 typically offers four times the cycle life. I doubt if any of us use our tanks sufficiently often to get anywhere near the expected life of a LiFePO4 battery..

http://www.armortek.co.uk/Forum3b/viewt ... f=2&t=5029

I don't have any doubt that LiFePO4 has significant advantages over lead acid (and I would never use LiPO) but my post was really about the apparent recovery characteristic, which I didn't really understand enough about.

Regards.

Stephen

[John Heath]

The apparent increase in voltage is called "relaxation effect" and is common in most types of batteries. It can be thought of as a normalisation of the remaining charge and doesn't increase the life of the battery as it's the charge settling so to speak.Most pronounced after heavily discharging a battery. Can be down to the chemicals involved or temperature or both. Google "battery relaxation effect" for more info.

An example is a torch that has stopped working but after turning it off for a while then turning it back on gives a dim light that doesn't last very long.

John

[Gerhard Michel]

The usable capacity of a battery and how to measure it is a never ending story! Lead acids increase there inner resistance, drop their load voltage and simulate a higher discharging than real exists. LiFes have a flat discharge 'line' and so simulate something more capacity than real available.

Therefore I spare any voltage measuring methods to dedect the used or remaining capacity by voltage, because this is much too inaccurate; see the mentioned recovery effects.

A modern 2.4 GHz R/C equipment offers the telemetry functions, and this is -in my eyes- the one and only true method to determine exactly the consumed capacity of a battery by using an integrating load sensor in the model (e.g. the Graupner EA module):



It pemanently gathers the voltage and the consumed current and calculates so the used capacity of the battery since starting (in mAh), and it shows that in a display on the transmitter (3rd line):



With a normal lead acid battery (AGM) one should only use half the nominal capacity (means 50% DoD). Therefore a 20 Ah AGM lead acid battery is 'empty' when the display shows 10 Ah! A lead silicone battery ('Greensaver') and a LiFe can be used up to 100 % DoD (= full nominal capacity) without leaving too much possible cycles in models which are used less than 30 times a year.

The telemetry sensor in combination with the transmitter has furthermore features as acoustic warning edges for voltages, capacity, current limits, temperatures and so on. So it is a nice and helpful tool for controlling a model's 'electrical behaviour' without permantently staring at the transmitter's display.