Laptop battery issues.

[LazyLand]

Hello!

I have a used Notebook Elitebook 840g1 and its battery is doing some wierd stuff. 
sometimes it shows that it has 10 precent but after plugging the power it suddenly have 50,

and somehow today instead of having like 4 hours of use i got only 1. it drained super fast. when I returned home I plugged it in and turned it on and it had more then it showed.. after like a few mins I don't remmber what I did but I just turned it on and I got a bios-battery error. (very low battery error) now after full charge it drained again super fast and but got a little bit more precent after restart. 

do you know whats going on maybe ?

also, I'm thinking of buying a second battery becuse In the hp help assistant it says that my model suppourt a second battery but, where do I put it? and where do it connect it?
also, should I buy a 30$ replecmeant or a 100$ original?

is there a cheaper original? becuase its wayy to much. 
it should be- cm03XL

thank you very much !!

[Huffer]

Your symptoms are consistent with an old tired battery that needs replacement:

6-cell, 60-Wh, 2.7-Ah (for use in EliteBook 740/840 and ZBook 14 models) 719796-001

Service Manual 

https://www.youtube.com/watch?v=4XfT4QaQ824

The secondary battery is a "slice" and connects via the docking port. They are very hard to find and make the laptop much larger to carry around. The number given above is for the "slice" battery.

https://www.ebay.co.uk/itm/HP-NEW-Original-Genuine-OEM-CS14C4-Slice-Battery-719796-001/253675981365?...

This is the regular internal battery:

3-cell, 50-Wh, 4.5-Ah 717376-001

https://www.ebay.com/itm/Genuine-HP-EliteBook-840-G1-HSTNN-IB4R-717376-001-CM03XL-Laptop-Battery-E7U...

Post back with any more questions and please accept as solution if this is the info you needed.