# making battery charger for lead acid battery 6v

Hello so I was just given a 6v 4.5 Ah (4500mAh) sealed lead acid battery. I have never made a battery charger before but would like to make one now for this battery as it is for a robot I'm building. How would I go about charging one of these batteries, as I have never done this before. Please correct me on this if I'm wrong but to me it seems that you can charge a lead acid battery by just connecting it to a voltage supply equivalent to what its voltage would be if it was fully charged and wait till the amperage pulled approaches 0. also as a side question I have another lead acid battery with the same specs but I have not used it and when i checked its voltage it was 2.8v. is this battery dead or can it be recharged and brought back to its previous state?

edit: how would I find the maximum current they battery can take?

edit 2: in the batteries spec sheet it said: "“Float” or “Stand-By” Service: Hold battery across constant voltage source of 6.75 to 6.90 volts continuously. When held at this voltage, the battery will seek its own current level and maintain itself in a fully charged condition". does this need current limitation? If so what would the name of that variable be or how could I find it?

Edit 3: So the quote in edit 2 is taken from the specsheet, so following there instruction can I give the battery 6.90V and it will seek its current and it does NOT have to be limited or is there still a current limitation requirement?

• Is it 4500 mAh (4.5 Ah) or 9 Ah (9000 mAh)? I would say your 2.8 volt battery is dead. Commented Mar 5, 2016 at 0:09
• Best method is to look up the actual battery model number and then follow the manufacturers recommended charging specifications. Some manufacturers even offer their own commercial battery chargers made specifically for their products.
– Nedd
Commented Mar 5, 2016 at 2:35
• For general reference, download & view the info from this datasheet: mouser.com/catalog/specsheets/884-4010.pdf Personally, during the CC charging phase, I would set charge current <= 1.6A, set the CV phase max V to 7V, and set the float V to 6.75V. If the batt V after 24h does not rest at or above 6.3V, then gradually increase your float V (1/4 V increments--but never exceeding 6.9V) until your 24h resting V = 6.3V. Commented Mar 5, 2016 at 19:34
• If your old batt never maintains 6.3V for 24h, then it is either hard-sulfated or it has micro-crystalline shorts between its plates--which makes it pretty much useless for most applications in either fault condition. Commented Mar 5, 2016 at 19:34