Leaf Spy AHr max by battery size

[alozzy]

Thanks SageBrush, a good explanation and a helpful analogy.

 

[goldbrick]

Another way to think of it is that the battery's capacity is equal to the area under the V-A curve. Since the battery's V drops as the capacity is used, the curve is not a rectangle. Using 360V as a nominal voltage makes the math easy since then the curve is a rectangle but is then only an approximation of the capacity. Remember W = V*A and in this case V is not constant but a function of SOC.

 

[alozzy]

You might just as well ask why "12 volt batteries" are rated that way, when they are more like "13 volt" batteries.

This is exactly what I was asking - why is the nominal voltage used as a reference for capacity. I wasn't talking about voltage, per se.

My confusion was with the formula kWh = V * Ah, as the empirical LeafSpy values didn't seem to work with that formula.

SageBrush's explanation was helpful...

I'm rusty on Integral calculus, but I think that the empirical capacity of the pack would be represented by the area under the discharge curve (V vs Ah). That can really only be derived empirically, by fully charging and the discharging the pack.

It bothers me that kWh is ever expressed by a linear equation (kWh = V * Ah), that's what was throwing me off...

 

[alozzy]

Another way to think of it is that the battery's capacity is equal to the area under the V-A curve. Since the battery's V drops as the capacity is used, the curve is not a rectangle. Using 360V as a nominal voltage makes the math easy since then the curve is a rectangle but is then only an approximation of the capacity. Remember W = V*A and in this case V is not constant but a function of SOC.

LOL, I was just putting that thought together, at the same time you were

 

[LeftieBiker]

If the same formula to determine capacity is used for all battery chemistries, then it will be wrong for several of them. With lead-acid you have to assume that no more than 50% of the calculated capacity is actually available for use. So a 12 volt, 50AH lead-acid battery is neither 12 volts nor 50AH - more like 13 volts and 25AH...

 

[SageBrush]

If the same formula to determine capacity is used for all battery chemistries, then it will be wrong for several of them. With lead-acid you have to assume that no more than 50% of the calculated capacity is actually available for use. So a 12 volt, 50AH lead-acid battery is neither 12 volts nor 50AH - more like 13 volts and 25AH...

Don't confuse the poor guy.

You may decide on some arbitrary Depth of discharge to extend longevity, but that has nothing to do with capacity.

 

[LeftieBiker]

I'm not talking about anything arbitrary. Peukert’s Law mean's that at higher rates of discharge, you get less capacity. In the case of lead-acid batteries, unless you are using the battery to power a few lights only, the available capacity will be about half of the rated capacity.

 

[SageBrush]

I'm not talking about anything arbitrary. Peukert’s Law mean's that at higher rates of discharge, you get less capacity. In the case of lead-acid batteries, unless you are using the battery to power a few lights only, the available capacity will be about half of the rated capacity.

I thought capacity tests are standardized to a 0.2C discharge rate.
For a 50 Ahr battery that is ~ 120 watts. Not much

 

[johnlocke]

I'm not talking about anything arbitrary. Peukert’s Law mean's that at higher rates of discharge, you get less capacity. In the case of lead-acid batteries, unless you are using the battery to power a few lights only, the available capacity will be about half of the rated capacity.

I thought capacity tests are standardized to a 0.2C discharge rate.
For a 50 Ahr battery that is ~ 120 watts. Not much

Actually it's even worse than that. Ratings for lead acid batteries are based on a 20 hour discharge period. For a 50 AH battery that's a 30w draw. Most automotive batteries are optimized for high current draw for short periods and a low recovery rate during recharge. That's why you don't see 50A battery chargers. 2-5A is the most common rate you see for the same reason.

 

[LeftieBiker]

Small EVs that use SLA batteries (even worse than flooded cell) should be - well, they should be equipped with lithium batteries, but if you use SLAs you assume 50% of rated capacity when brand new, with rapid decline. Over at EndlesSphere.com we usually encourage people to avoid SLA altogether, for applications larger than a child's toy ride-on vehicle.

 

[SageBrush]

I'm not talking about anything arbitrary. Peukert’s Law mean's that at higher rates of discharge, you get less capacity. In the case of lead-acid batteries, unless you are using the battery to power a few lights only, the available capacity will be about half of the rated capacity.

I thought capacity tests are standardized to a 0.2C discharge rate.
For a 50 Ahr battery that is ~ 120 watts. Not much

Actually it's even worse than that. Ratings for lead acid batteries are based on a 20 hour discharge period. For a 50 AH battery that's a 30w draw. Most automotive batteries are optimized for high current draw for short periods and a low recovery rate during recharge. That's why you don't see 50A battery chargers. 2-5A is the most common rate you see for the same reason.

Good to know -- thanks.
Yeah ... normal use of say non LED lights does sounds like multiples of a 0.05C discharge test.

Do you know what the charging Amps are when the LEAF is using the traction battery to charge a 12v at e.g. 50% SoC ?

 

[GerryAZ]

The LEAF's DC-DC converter provides approximately 14-14.5 volts until the current drops below a threshold and then drops back to float voltage of about 13 volts (depending upon ambient temperature) whether the car is in ready mode or the traction battery is being charged. It also charges the 12V battery from the traction battery periodically while the car is parked without being plugged in. The threshold value depends upon the model year of the LEAF (my 2011 was 6-8 amperes, 2015 was 3-4 amperes, and 2019 is 1.5-2 amperes). If the 12V battery is in good condition with relatively low internal resistance, the current will be quite high since the DC-DC converter is capable of well over 100 amperes.

 

[SageBrush]

The LEAF's DC-DC converter provides approximately 14-14.5 volts until the current drops below a threshold and then drops back to float voltage of about 13 volts (depending upon ambient temperature) whether the car is in ready mode or the traction battery is being charged. It also charges the 12V battery from the traction battery periodically while the car is parked without being plugged in. The threshold value depends upon the model year of the LEAF (my 2011 was 6-8 amperes, 2015 was 3-4 amperes, and 2019 is 1.5-2 amperes). If the 12V battery is in good condition with relatively low internal resistance, the current will be quite high since the DC-DC converter is capable of well over 100 amperes.

Thanks -- interesting.

I'll ask the obvious question: why does the new 2019 battery have ~ 3 - 4x the resistance of the 2011 model ?

 

[DaveinOlyWA]

I thought capacity tests are standardized to a 0.2C discharge rate.
For a 50 Ahr battery that is ~ 120 watts. Not much

Actually it's even worse than that. Ratings for lead acid batteries are based on a 20 hour discharge period. For a 50 AH battery that's a 30w draw. Most automotive batteries are optimized for high current draw for short periods and a low recovery rate during recharge. That's why you don't see 50A battery chargers. 2-5A is the most common rate you see for the same reason.

Good to know -- thanks.
Yeah ... normal use of say non LED lights does sounds like multiples of a 0.05C discharge test.

Do you know what the charging Amps are when the LEAF is using the traction battery to charge a 12v at e.g. 50% SoC ?

After a 6 week review of monitoring 12 volt charging at morning startup, the current ranges from 2.96 to 1.7ish amps. The charge appears to be on a timer so not the typical 12 volt charging pattern. I have seen charges end with current anywhere from the 1.7 to as much as 2.3 amp. All the charges I monitored ran 4-6 minutes.

 

[SageBrush]

After a 6 week review of monitoring 12 volt charging at morning startup, the current ranges from 2.96 to 1.7ish amps. The charge appears to be on a timer so not the typical 12 volt charging pattern. I have seen charges end with current anywhere from the 1.7 to as much as 2.3 amp. All the charges I monitored ran 4-6 minutes.

2 amps for 5 minutes is a whopping 0.16 Wh
Even a modern cell phone would sneer.

 

[DaveinOlyWA]

After a 6 week review of monitoring 12 volt charging at morning startup, the current ranges from 2.96 to 1.7ish amps. The charge appears to be on a timer so not the typical 12 volt charging pattern. I have seen charges end with current anywhere from the 1.7 to as much as 2.3 amp. All the charges I monitored ran 4-6 minutes.

2 amps for 5 minutes is a whopping 0.16 Wh
Even a modern cell phone would sneer.

Yep. To compare, I put a 4 amp charger on it and it took about 3-4 mins to get to 50%, an additional 13 mins to get to 75%

So this past Friday, I check the battery and its 11.88 volts so I put the charger on and this time it took 6 mins to get to 25% (My charger only increments 25%) and another 13-15 mins (sorry didn't stand out there watching it) to get to 50% so yeah; for the most part, the battery is barely surviving most of the winter.

 

[SageBrush]

Yep. To compare, I put a 4 amp charger on it and it took about 3-4 mins to get to 50%, an additional 13 mins to get to 75%

So this past Friday, I check the battery and its 11.88 volts so I put the charger on and this time it took 6 mins to get to 25% (My charger only increments 25%) and another 13-15 mins (sorry didn't stand out there watching it) to get to 50% so yeah; for the most part, the battery is barely surviving most of the winter.

I don't think your measurements make sense.

A new battery is around 40 Ahr
You are talking about ~ ONE Ahr capacity

Perhaps you are measuring surface charge. Gerry will know

 

[DaveinOlyWA]

Yep. To compare, I put a 4 amp charger on it and it took about 3-4 mins to get to 50%, an additional 13 mins to get to 75%

So this past Friday, I check the battery and its 11.88 volts so I put the charger on and this time it took 6 mins to get to 25% (My charger only increments 25%) and another 13-15 mins (sorry didn't stand out there watching it) to get to 50% so yeah; for the most part, the battery is barely surviving most of the winter.

I don't think your measurements make sense.

A new battery is around 40 Ahr
You are talking about ~ ONE Ahr capacity

Perhaps you are measuring surface charge. Gerry will know

My SOC statements are based on the display on the charger itself. Have no idea what the accuracy is but guessing its not very. Another time I put it on the charger when it was 11.91 volts (connected to car) and it took over 5 hours to get it to a full charge. I didn't time the %'s then

 

[SageBrush]

My SOC statements are based on the display on the charger itself. Have no idea what the accuracy is but guessing its not very. Another time I put it on the charger when it was 11.91 volts (connected to car) and it took over 5 hours to get it to a full charge. I didn't time the %'s then

They could have come from the FSM itself. Surely you can see that they do not make sense.

 

[GerryAZ]

The LEAF's DC-DC converter provides approximately 14-14.5 volts until the current drops below a threshold and then drops back to float voltage of about 13 volts (depending upon ambient temperature) whether the car is in ready mode or the traction battery is being charged. It also charges the 12V battery from the traction battery periodically while the car is parked without being plugged in. The threshold value depends upon the model year of the LEAF (my 2011 was 6-8 amperes, 2015 was 3-4 amperes, and 2019 is 1.5-2 amperes). If the 12V battery is in good condition with relatively low internal resistance, the current will be quite high since the DC-DC converter is capable of well over 100 amperes.

Thanks -- interesting.

I'll ask the obvious question: why does the new 2019 battery have ~ 3 - 4x the resistance of the 2011 model ?

The internal resistances of the OEM 12V batteries in all 3 cars are (were) similar. The AGM replacements in the 2011 and 2015 were much lower so they would recharge faster. The current threshold when the charging voltage drops back to float is controlled by the car (the lower threshold on the later cars means the 12V battery gets to a higher SOC before the DC-DC converter drops back to float). The Hall effect device at the negative terminal provides battery charging current (absorption current) signal to the control modules in the car. You can see that current and battery voltage displayed in Leaf Spy.

I have never see voltages as low as Dave posted and have never connected an external 12V charger.