Maximum and minimum SLA battery voltages

[revans]

Hey there, for this circuit here: **broken link removed** (a 12v SLA battery monitor) I need to know the maximum and minimum voltages of the battery I'm using (essentially its voltage when fully charged and its voltage when flat).

The circuit is already preset for a range between 11.89-12.65 volts. However, the battery I have (this one: https://www.electro-tech-online.com/custompdfs/2008/12/20085271025189043-2.pdf) has a fully charged voltage of 13.15v (I think...this could be wrong, but it's at least this much), so I will need to adjust resistors at pin 4 as per the circuit's blurb on the website.

Unfortunately, I have no idea what the "flat" voltage of the battery is and do not want/can't be bothered discharging it to measure it (especially as I was under the impression it's bad to fully discharge SLAs). Would anyone be able to tell me or hazard a fairly accurate guess at the minimum voltage of my battery? Thanks.

 

[Leftyretro]

A SLA battery have the same voltage characteristics as a standard lead-acid battery.

Here is a chart showing terminal voltage Vs state of charge. Note that the reading you get while on a charger or even after removing the charger are not valid, until the battery has had a chance to settle. The chart suggest waiting 3 hours after a complete charge to take a reading:

LEAD ACID BATTERY CHARGING INFORMATION, BATTERY TYPES AND OPERATION - SOLAR NAVIGATOR WORLD ELECTRIC NAVIGATION CHALLENGE, NELSON KRUSCHANDL, BLUEBIRD ELECTRIC LAND SPEED RECORD CARS

 

[revans]

Thanks lefty. The guy I bought the battery from said he charged it fully before I received it. Is this why it may have been at 13.15v? Because of the time required to "settle"?

Essentially then I should still be using the standard voltage interval of 11.89-12.65v the circuit is made for. Correct?

 

[Leftyretro]

Thanks lefty. The guy I bought the battery from said he charged it fully before I received it. Is this why it may have been at 13.15v? Because of the time required to "settle"?

Essentially then I should still be using the standard voltage interval of 11.89-12.65v the circuit is made for. Correct?

Correct, those are good values to use.

 

[revans]

Ok I just measured the voltage of my battery this morning, figuring it should have "settled" to a voltage more like the 12.7v as per above posts.

However, the voltage was still 13.1v, so evidently, this battery doesn't have the normal range of 11.89-12.65

Can anyone help me further with the minimum voltage of an SLA battery that has a fully charged voltage of 13.15v?

 

[kchriste]

The link to the datasheet that you posted can help you figure this out. According the "discharge curves", it appears that the battery is TOTALLY discharged when it reaches 10.8V. To preserve the battery's life, you don't want to go that low. If you are not going to discharge it at a rate greater than 5A at 25 degC, then I would use 12.0V as the "empty" point for this battery.

 

[revans]

Ok thanks for that kchriste. 11.89v as empty should be alright instead of 12v though, right? The circuit is preset for this and if I use 11.89 I don't need to change any resistors.

 

[Leftyretro]

Ok thanks for that kchriste. 11.89v as empty should be alright instead of 12v though, right? The circuit is preset for this and if I use 11.89 I don't need to change any resistors.

Yes, 11.89 is close to the 11.9 which is that last green line in the chart saying that 40% charge is left. This is the recommend lowest discharge level for longest life of battery. Think of it as a gas guage in a car where the gauge drops down to red, but there is still some small remaining fuel for emergency use. The chart talks about using just the top 60% of charge capacity of the battery for best overall use of the battery.