How do i full charge the black & decker electromate 400

it's not a computer is this,
this tool is for cars, battery charge and jump. 11 years old
read page 6
https://www.manualslib.com/manual/16290/Black-And-Decker-Electromate-90556511.html?page=6#manual

this old toll has and old AGM battery inside
none made last this long ever, and if not kept then internal battery charged it will go bad in 6 months due to sulfation. !

Should only be about an hour depending on what battery you have.

here is a link to the instructions that came with your jump box:
https://www.manualslib.com/manual/17003/Black-And-Decker-Electromate-Vec027bd.html?page=4#manual

contact Black and decker i am sure they have some.

This link just might work for you: http://servicenet.blackanddecker.com/documents/English/Instruction%20Manual/90556511,VEC026BD.pdf

Just to play it safe I would NOT jump start while charging, It sounds like one would be putting out too much volts/amps ETC and could damage or the even kill the battery.

battery may be dead and need replacement

Did you try this?
Fuse Replacement (DC Accessory Adapter)
1. Remove plug from accessory outlet. Remove the gold cap by turning counterclockwise
and lifting off.
2. Remove center pin and spring. Remove fuse.
3. Replace fuse with same type and size fuse (8 amp).
4. Replace center pin and spring inside plug.
5. Replace gold cap by turning clockwise.
For assistance with your product, visit our website www.blackanddecker.com for the
location of the service center nearest you or call the BLACK & DECKER help line
at 1-800-544-6986.

I went to the Black & Decker site and downloaded the pdf manual of the electromate 400. It says you're supposed to charge it until you have one red light, one yellow light, and one green light. I expect that 2 red lights and a green light means it's not fully charged. The manual also said you can't overcharge using the 120 V. method. Good luck.

Why does the AC indicator light constantly flash and I have no AC power on my Black and Decker Electormate 400