[SOLVED] UC3909 charge algorithm (cc + cv) not accepting constant voltage battery

[electrophysics]

Hi,
I made a SLA battery charger using my designed offline flyback SMPS and implemented charge algorithm using UC3909. Meanwhile I shopped a 7 Ahr battery from local market. Surprisingly when I read the label on battery, it was constant voltage type with cyclic use from 14.4 V to 14.7 V and standby use 13.4 V to 13.7 V. UC3909 operates both as Constant Current, CC and Constant Voltage, CV depending on the state of battery voltage and current demand. I made the circuit almost same as in the link below,

**broken link removed**

I think constant voltage battery can not operate in constant current part of algorithm. However when searched the market after concluding this then another surprise came while reading labels on SLA batteries that they all were constant voltage type.

Is it really that way?

 

[FvM]

I think constant voltage battery can not operate in constant current part of algorithm.

Much confusion in a short sentence. Never saw a thing like a "constant voltage battery". And even if it would exist, why can't it be charged with constant current?

Sealed lead-acid (SLA) batteries are basically all the same. They have usually different voltage specifications for cycle and standby (float or trickle charge) operation. Constant current will be applied during the inital charge phase in both cases.

A dedicated charger IC like UC3909 is trying to charge the battery as much as possible without reducing the lifetime by keeping a high voltage continuously. It can be also used in standby application if the battery current is sensed separately.

 

[electrophysics]

Much confusion in a short sentence. Never saw a thing like a "constant voltage battery". And even if it would exist, why can't it be charged with constant current?

Be a grown up man. I mean a battery having charge mechanism of applying constant voltage. Please read about the battery type as below link if you are innocent enough about not knowing it,

https://www.panasonic.com/industrial/includes/pdf/Panasonic_VRLA_ChargingMethods.pdf

I have searched some time in past about chips that work like UC3909 for SLA but could not find more than couple of them. That's another issue. There are some batteries I saw on net that support both constant voltage and constant current (like above link file indicates). That might be the right candidate for UC3909. In constant voltage charge mechanism it is straight forward technique to fix the upper threshold of SMPS to 14.7 V and by fixing the maximum current via some sense resistor to value as specified on battery label written as "initial charge current" it would charge it. Then after couple of hours seeing that current level has dropped too much it can be switched to another constant voltage of 13.7 V for float level then onwards.
If you try to use this chip on constant voltage charge mechanism type battery, its voltage rises on battery terminal directly to 14.7 V instead of increasing slowly according to constant bulk current and charge level voltage. I mean this skips the bulk charge part. If you see the application note of TI that I attached the link you will clearly see that there would be much time for the battery to behave as constant bulk current before reaching to overcharge threshold of 14.7 V.

 

[FvM]

Among other things, I'm designing SLA battery chargers since about 40 years, but I simply don't understand your problem.

 

[electrophysics]

Dear FvM. Don't take it personally. My problem that I don't involve in politics and so being singled out from local colleagues and running electronic switching products design consultancy for last five years with zero finances.

 

[FvM]

It's just a technical comment. I don't understand the problem. And to keep you from jumping into conclusions, I wanted to clarify that I'm not new to battery charging.

All SLA batteries can be charged with CC/CV method. The problem is about the optimal voltage level. UC3909 solves it by applying two voltage levels. I don't see why any SLA battery couldn't be charged with this method. Don't misunderstand a CC/CV characteristic as a required timing diagram.

If you try to use this chip on constant voltage charge mechanism type battery, its voltage rises on battery terminal directly to 14.7 V instead of increasing slowly according to constant bulk current and charge level voltage.

If the voltage rises immediately to charge end voltage with specified current, either the battery is full or has reached it's end of life.

 

[electrophysics]

If the voltage rises immediately to charge end voltage with specified current, either the battery is full or has reached it's end of life.

Instead of controlling constant bulk current to initial maximum charge current written on label as 2.8 A for 7 AHr battery, I lowered the constant bulk current to 0.25 CA = 1.7 A as was shown for the tests in its datasheet graph by adjusting the resistor for bulk current and got the solution. That graph in datasheet was just like constant voltage + constant current. When measured current it was around 1.7 A constant as predicted.