SLA Battery State of Charge...

[Rick C]

I\'ve been looking for data on determining state of charge of an SLA battery based on voltage. The data seems to be all over the map. There is some variation from the specifics of the battery type, flooded SLA, AGM and gel battery. But even then there are numbers on the web that are all over the map.

I\'m guessing this is because the limits of what is \"full\" and what is \"empty\" depend on how much you wish to baby the battery and how much capacity you wish to obtain from it.

We have to different needs. One is to provide a battery gauge showing the charge status. The other is to sound levels of alarms for running low and nearly out of remaining run time.

Anyone have experience with this?

--

Rick C.

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[Bill Sloman]

On Tuesday, December 8, 2020 at 5:38:07 PM UTC+11, gnuarm.del...@gmail.com wrote:

I\'ve been looking for data on determining state of charge of an SLA battery based on voltage. The data seems to be all over the map. There is some variation from the specifics of the battery type, flooded SLA, AGM and gel battery. But even then there are numbers on the web that are all over the map.

I\'m guessing this is because the limits of what is \"full\" and what is \"empty\" depend on how much you wish to baby the battery and how much capacity you wish to obtain from it.

We have to different needs. One is to provide a battery gauge showing the charge status. The other is to sound levels of alarms for running low and nearly out of remaining run time.

Anyone have experience with this?

Very little. The theory is simple enough - the output voltage is proportional to the logarithm of of the concentration of the charged element over the concentration of the discharged element.

Unfortunately the constant of proportionality varies with temperature, so you have to know the temperature of the relevant bit of the battery to work it out.

Charging or discharging a battery makes it warmer, and the heat takes time to diffuse away from the area between the plates where it gets generated.

In theory you could put a thermistor inside the battery, but nobody does.

Pumping in or taking out a little charge, and measuring how far the battery voltage moves per quantum of charge could be more informative, and doing it for a bit in alternation would mean that you were always dumping in the same amount of heat while it was going on, and have the chance the measure a steady temperature rise at the surface of the battery.

It\'s still pretty horrid.

--
Bill Sloman, Sydney

 

[Mike Perkins]

On 08/12/2020 06:38:00, Rick C wrote:

I\'ve been looking for data on determining state of charge of an SLA battery based on voltage. The data seems to be all over the map. There is some variation from the specifics of the battery type, flooded SLA, AGM and gel battery. But even then there are numbers on the web that are all over the map.

I\'m guessing this is because the limits of what is \"full\" and what is \"empty\" depend on how much you wish to baby the battery and how much capacity you wish to obtain from it.

We have to different needs. One is to provide a battery gauge showing the charge status. The other is to sound levels of alarms for running low and nearly out of remaining run time.

Anyone have experience with this?

There are some smart devices that measure internal cell resistance, and
from that and a variation of Peukert\'s law get an indication of residual
charge.

The usual method is to check terminal voltage and make an educated guess.

Most alarms are set by a terminal voltage of less that 11V, or perhaps a
little higher. Obviously dependent on discharge rate and Peukert\'s law!

https://en.wikipedia.org/wiki/Peukert%27s_law

--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

 

[Rick C]

On Wednesday, December 9, 2020 at 7:46:05 PM UTC-5, Mike Perkins wrote:

On 08/12/2020 06:38:00, Rick C wrote:
I\'ve been looking for data on determining state of charge of an SLA battery based on voltage. The data seems to be all over the map. There is some variation from the specifics of the battery type, flooded SLA, AGM and gel battery. But even then there are numbers on the web that are all over the map.

I\'m guessing this is because the limits of what is \"full\" and what is \"empty\" depend on how much you wish to baby the battery and how much capacity you wish to obtain from it.

We have to different needs. One is to provide a battery gauge showing the charge status. The other is to sound levels of alarms for running low and nearly out of remaining run time.

Anyone have experience with this?
There are some smart devices that measure internal cell resistance, and
from that and a variation of Peukert\'s law get an indication of residual
charge.

The usual method is to check terminal voltage and make an educated guess.

Most alarms are set by a terminal voltage of less that 11V, or perhaps a
little higher. Obviously dependent on discharge rate and Peukert\'s law!

https://en.wikipedia.org/wiki/Peukert%27s_law

Thanks for the suggestion, but I don\'t follow. Peukert\'s law relates discharge rate to usable capacity in the battery. I don\'t see anything relating to remaining capacity.

Our design will drive intermittent high currents so if the voltage differential can say something about the remaining capacity, we might be able to use that.

--

Rick C.

+ Get 1,000 miles of free Supercharging
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[Martin Brown]

On 10/12/2020 01:20, Rick C wrote:

On Wednesday, December 9, 2020 at 7:46:05 PM UTC-5, Mike Perkins
wrote:
On 08/12/2020 06:38:00, Rick C wrote:

We have to different needs. One is to provide a battery gauge
showing the charge status. The other is to sound levels of alarms
for running low and nearly out of remaining run time.

Anyone have experience with this?
There are some smart devices that measure internal cell resistance,
and from that and a variation of Peukert\'s law get an indication of
residual charge.

The usual method is to check terminal voltage and make an educated
guess.

Most alarms are set by a terminal voltage of less that 11V, or
perhaps a little higher. Obviously dependent on discharge rate and
Peukert\'s law!

https://en.wikipedia.org/wiki/Peukert%27s_law

Thanks for the suggestion, but I don\'t follow. Peukert\'s law relates
discharge rate to usable capacity in the battery. I don\'t see
anything relating to remaining capacity.

Remaining capacity is a function of the rate of discharge so it affects
the remaining capacity - although this may be irrelevant if the most
important thing is to be able to drive a moderately high current.

Our design will drive intermittent high currents so if the voltage
differential can say something about the remaining capacity, we might
be able to use that.

Monitoring that drop in voltage under maximum load and an empirical
determination might be the simplest way of doing it. You probably want a
two level warning of <30 minutes remaining and a 10 minute warning.

Will it have provision for connecting up an external (eg car) battery or
hot swapping a new battery pack in like some bigger UPS\'s do ?

--
Regards,
Martin Brown

 

[Phil Allison]

Martin Brown wrote:
==================

Remaining capacity is a function of the rate of discharge so it affects
the remaining capacity -

** Absolutely - for any SLA battery.

Discharging at 10C results in a capacity loss of almost 90%.
Don\'t matter if it\'s intermittent or continuous.

The OP here is a well known, fucking idiot.
Believes just whatever he feels like believing.
Espouses any idea that gives him a quick hard-on.

Massive Biden supporter - naturally....


...... Phil

 

[bob prohaska]

Rick C <gnuarm.deletethisbit@gmail.com> wrote:

Our design will drive intermittent high currents so if the voltage differential can say something about the remaining capacity, we might be able to use that.

It\'s long been my impression the the exact voltage of a lead-acid battery
is an artifact of construction. For example, lead-calcium batteries seem
to display slightly higher voltage than lead-antimony.

It does seem you could extract some hint of SOC from the ratio of loaded
to unloaded voltage. Useless in a float situation, unfortunately. In any
case, I believe you\'ll have to know something about the individual battery,
or at least the battery lot/type/chemistry.

hth,

bob prohaska

 

[Rick C]

On Thursday, December 10, 2020 at 4:38:05 AM UTC-5, Martin Brown wrote:

On 10/12/2020 01:20, Rick C wrote:
On Wednesday, December 9, 2020 at 7:46:05 PM UTC-5, Mike Perkins
wrote:
On 08/12/2020 06:38:00, Rick C wrote:

We have to different needs. One is to provide a battery gauge
showing the charge status. The other is to sound levels of alarms
for running low and nearly out of remaining run time.

Anyone have experience with this?
There are some smart devices that measure internal cell resistance,
and from that and a variation of Peukert\'s law get an indication of
residual charge.

The usual method is to check terminal voltage and make an educated
guess.

Most alarms are set by a terminal voltage of less that 11V, or
perhaps a little higher. Obviously dependent on discharge rate and
Peukert\'s law!

https://en.wikipedia.org/wiki/Peukert%27s_law

Thanks for the suggestion, but I don\'t follow. Peukert\'s law relates
discharge rate to usable capacity in the battery. I don\'t see
anything relating to remaining capacity.
Remaining capacity is a function of the rate of discharge so it affects
the remaining capacity - although this may be irrelevant if the most
important thing is to be able to drive a moderately high current.
Our design will drive intermittent high currents so if the voltage
differential can say something about the remaining capacity, we might
be able to use that.
Monitoring that drop in voltage under maximum load and an empirical
determination might be the simplest way of doing it. You probably want a
two level warning of <30 minutes remaining and a 10 minute warning.

Will it have provision for connecting up an external (eg car) battery or
hot swapping a new battery pack in like some bigger UPS\'s do ?

The <30 minute warning will come on as soon as we switch to battery, lol. I expect 30 minute run time on battery, the spec calls for 20. The idea is to give time to make other arrangements, not to be able to run indefinitely.

It\'s not my call.

I see our resident meths drinker has replied providing no useful content, just the usual ranting. 10C discharge would be a rather fast run down rate indeed. Good thing we aren\'t doing that.

I hope his delirium tremors get better.

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209

 

[Rick C]

On Thursday, December 10, 2020 at 11:49:46 AM UTC-5, bob prohaska wrote:

Rick C <gnuarm.del...@gmail.com> wrote:

Our design will drive intermittent high currents so if the voltage differential can say something about the remaining capacity, we might be able to use that.

It\'s long been my impression the the exact voltage of a lead-acid battery
is an artifact of construction. For example, lead-calcium batteries seem
to display slightly higher voltage than lead-antimony.

It does seem you could extract some hint of SOC from the ratio of loaded
to unloaded voltage. Useless in a float situation, unfortunately. In any
case, I believe you\'ll have to know something about the individual battery,
or at least the battery lot/type/chemistry.

Maybe I\'m wrong, but I thought I mentioned it is an AGM type, no? Even so, I find different voltage data and the voltage also depends on temperature and I can\'t get the power board designer to acknowledge putting a temperature sensor on the power board (which will be some 10°C warming than ambient) is useless.

--

Rick C.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209

 

[bob prohaska]

Rick C <gnuarm.deletethisbit@gmail.com> wrote:

On Thursday, December 10, 2020 at 11:49:46 AM UTC-5, bob prohaska wrote:
Rick C <gnuarm.del...@gmail.com> wrote:

Our design will drive intermittent high currents so if the voltage differential can say something about the remaining capacity, we might be able to use that.

It\'s long been my impression the the exact voltage of a lead-acid battery
is an artifact of construction. For example, lead-calcium batteries seem
to display slightly higher voltage than lead-antimony.

It does seem you could extract some hint of SOC from the ratio of loaded
to unloaded voltage. Useless in a float situation, unfortunately. In any
case, I believe you\'ll have to know something about the individual battery,
or at least the battery lot/type/chemistry.

Maybe I\'m wrong, but I thought I mentioned it is an AGM type, no? Even so, I find different voltage data and the voltage also depends on temperature and I can\'t get the power board designer to acknowledge putting a temperature sensor on the power board (which will be some 10?C warming than ambient) is useless.

I didn\'t catch it was AGM. That probably rules out lead-antimony.

How much will be known about the battery deployed? If nothing more
than it\'s a lead-acid of N volts and P amp-hours I think it\'ll be
infeasible to fashion a better than very crude \"gas gauge\". If the
designer will specify a particular product, it might be useful
to get a few samples and characterize them. No help at all as
things age, unfortunately. Any chance of an integrating ammeter?

Good luck,

bob prohaska

 

[Edward Rawde]

\"bob prohaska\" <bp@www.zefox.net> wrote in message
news:rqueta$pkc$1@dont-email.me...

Rick C <gnuarm.deletethisbit@gmail.com> wrote:
On Thursday, December 10, 2020 at 11:49:46 AM UTC-5, bob prohaska wrote:
Rick C <gnuarm.del...@gmail.com> wrote:

Our design will drive intermittent high currents so if the voltage
differential can say something about the remaining capacity, we might
be able to use that.

It\'s long been my impression the the exact voltage of a lead-acid
battery
is an artifact of construction. For example, lead-calcium batteries seem
to display slightly higher voltage than lead-antimony.

It does seem you could extract some hint of SOC from the ratio of loaded
to unloaded voltage. Useless in a float situation, unfortunately. In any
case, I believe you\'ll have to know something about the individual
battery,
or at least the battery lot/type/chemistry.

Maybe I\'m wrong, but I thought I mentioned it is an AGM type, no? Even
so, I find different voltage data and the voltage also depends on
temperature and I can\'t get the power board designer to acknowledge
putting a temperature sensor on the power board (which will be some 10?C
warming than ambient) is useless.


I didn\'t catch it was AGM. That probably rules out lead-antimony.

How much will be known about the battery deployed? If nothing more
than it\'s a lead-acid of N volts and P amp-hours I think it\'ll be
infeasible to fashion a better than very crude \"gas gauge\". If the
designer will specify a particular product, it might be useful
to get a few samples and characterize them. No help at all as
things age, unfortunately.

Any chance of an integrating ammeter?

That should have been done from the start.
I wonder whether his spec specifies how often battery replacement should be
carried out.

Good luck,

bob prohaska

 

[Rick C]

On Thursday, December 10, 2020 at 9:41:11 PM UTC-5, Edward Rawde wrote:

\"bob prohaska\" <b...@www.zefox.net> wrote in message
news:rqueta$pkc$1...@dont-email.me...
Rick C <gnuarm.del...@gmail.com> wrote:
On Thursday, December 10, 2020 at 11:49:46 AM UTC-5, bob prohaska wrote:
Rick C <gnuarm.del...@gmail.com> wrote:

Our design will drive intermittent high currents so if the voltage
differential can say something about the remaining capacity, we might
be able to use that.

It\'s long been my impression the the exact voltage of a lead-acid
battery
is an artifact of construction. For example, lead-calcium batteries seem
to display slightly higher voltage than lead-antimony.

It does seem you could extract some hint of SOC from the ratio of loaded
to unloaded voltage. Useless in a float situation, unfortunately. In any
case, I believe you\'ll have to know something about the individual
battery,
or at least the battery lot/type/chemistry.

Maybe I\'m wrong, but I thought I mentioned it is an AGM type, no? Even
so, I find different voltage data and the voltage also depends on
temperature and I can\'t get the power board designer to acknowledge
putting a temperature sensor on the power board (which will be some 10?C
warming than ambient) is useless.


I didn\'t catch it was AGM. That probably rules out lead-antimony.

How much will be known about the battery deployed? If nothing more
than it\'s a lead-acid of N volts and P amp-hours I think it\'ll be
infeasible to fashion a better than very crude \"gas gauge\". If the
designer will specify a particular product, it might be useful
to get a few samples and characterize them. No help at all as
things age, unfortunately.

Any chance of an integrating ammeter?
That should have been done from the start.
I wonder whether his spec specifies how often battery replacement should be
carried out.

The integrating charge measurement could easily be done as the FPGA will monitor the current. But that\'s like a parachute jump with a meter telling you your distance from the airplane. How close you are to the ground depends on how high the plane was when you jumped. Over temperature and age that distance will change. So the crude voltage measurement will have to suffice.

It keeps coming back to the team lead thinking his opinion should be the guiding light even when he knows little about a topic. I\'ll be honest and admit I don\'t know a lot about batteries, but I can read and have read everything I can find. That\'s why it is apparent to me that there is not a level of consistency that will allow a very accurate gas gauge on the remaining time for the battery. I would not worry with it myself... but I\'m responsible for the alarms and once the power fails the battery has three states determining the level of alarm. We discussed this a bit today and will need to wing it. The lead pointed out that we don\'t have to make a decision today and I pointed out that we won\'t know any more about the matter tomorrow. Putting off many things, such as creating requirements and analyzing errors in measurements is the sort of thing that needs to be done up front, not after someone has spent weeks or months working on something.

Virtually every part of this project has been done the way kids might develop some gadget based on an Arduino. In fact, that was the original design. Then they cobbled together some bits to control a motor and connected a pressure sensor. Adding bit by bit they got something that looks like it works, but it doesn\'t come anywhere near working in a real manner.

I do get a laugh from time to time. Every time we need to add some electronics I make a mental bet with myself that the guy doing the power module will find a Linear Technology part to use... so far I\'ve always won that bet. There is nothing in the LT catalog he doesn\'t love. $$$ but he doesn\'t seem to care. Heck, he had one part on the board that was pretty much superfluous. It was a input power monitor that cut the power if out of range or reversed. But everything else was LT and already surviving to nearly the same voltages and reverse.

He still has not produced an off state power budget. The LT parts might do a good job, but they are going to drain the battery faster than we would like. We also are going to see current surges on the large motor caps that they are not rated for. I\'m going to have to bring that up again in the design review.

However, it is important to keep in mind that we are not designing a product. What we are creating is more of a reference design. It will be up to a manufacturer to modify the design to suit their requirements. I just know I\'m not going to design in anything that has a chance of harming a patient.. This battery issue is sounding more and more like one of those things that can not be done adequately. So the alarm will just be one state, the highest since the potential harm to the patient is severe if the machine shuts off. If they want the design to measure potentially inaccurate battery levels and lower states of alarm, they will need to have someone else do it..

--

Rick C.

+- Get 1,000 miles of free Supercharging
+- Tesla referral code - https://ts.la/richard11209

 

[Edward Rawde]

\"Rick C\" <gnuarm.deletethisbit@gmail.com> wrote in message
news:3a211ddb-a513-4e4e-9707-0a88863e8d56n@googlegroups.com...
On Thursday, December 10, 2020 at 9:41:11 PM UTC-5, Edward Rawde wrote:

\"bob prohaska\" <b...@www.zefox.net> wrote in message
news:rqueta$pkc$1...@dont-email.me...
Rick C <gnuarm.del...@gmail.com> wrote:
On Thursday, December 10, 2020 at 11:49:46 AM UTC-5, bob prohaska
wrote:
Rick C <gnuarm.del...@gmail.com> wrote:

Our design will drive intermittent high currents so if the voltage
differential can say something about the remaining capacity, we
might
be able to use that.

However, it is important to keep in mind that we are not designing a
product.

That\'s a relief. In that case I will try to stop my brain coming up with
parallels to the 737 MAX design process.

--

Rick C.