Does floated lead-acid battery also self limit the current?

[harvie]

Hello,
i've been reading lot of stuff about lead-acid batteries and float charging of them, however i can't figure one thing... Can i be sure that under no circumstances the battery will take more current than it should?

For example:

I have typical 12V 7Ah battery with datasheet stating 13.6-13.8V float voltage and 2.16A maximum charging current.

Does this also mean that this battery will never take more than 2.16A from 13.6V supply even when it will be discharged very very hard? like discharged to 9.5V or something...

I plan to put 2.5-3A resetable (poly)fuse in series with that battery. But i want to be sure that fuse will not trigger itself when battery is OK. It will be there just for cases of internal short circuit in the battery, so it will not short the power supply... but i don't want the fuse to prevent it from charging when it's not completely dead and rechaging is still possible.

Also i don't want vital (but heavily discharged) battery to kill itself by overcurrent caused by applying float voltage to completely flat battery... As datasheet says 2.16A is the limit...

Discharged battery should have 10.8V... So i subtract that from float voltage to get voltage difference between supply and battery 13.6-10.8=2.8V. Datasheet says batetry has internal resistance of 19 mOhm. So let's apply ohms law:

2.8V/0.019ohm = 147.3684 A

That's pretty scary! Or am i missing something?


So do i need to limit the current further or float voltage (with proper thermal compensation) is enough to keep the cell current within the 2.16A? THX.

 

[esp1]

hi,
Ideally the Charger should have an inbuilt maximum current limit of 2.16Amp and also a maximum charge voltage of 13.6v to 13.8V.

I would advise you do not allow a discharged SLA battery voltage fall below 11.0V.

E

 

[harvie]

hi,
Ideally the Charger should have an inbuilt maximum current limit of 2.16Amp

I know... Ideally you would use smarter charger with more than one charging phase...

But what can happen if it does not have such limit? most of the articles about float charging are completely omitting current limits... They even say that battery will "magically draw just enough", but no exact definition...

 

[D.A.(Tony)Stewart]

You can search this site or anywhere to get ideal charger profile.

In almost all cases, the Charger is responsible for constant Voltage limit, (CV) constant current limit (CC) and drop to float voltage (FV)
... not the battery.

CV is normally 14.2V.
CC was given.
FV is normally 13.6.

There are temperature corrections.