I have a 15.6v NiCad metabo drill can you replace 15.6v battery with the 18v which fits exactly the same in the drill

Question edited for a lot more clarity. All questions need full 'make model (what it is)' please.
Question moved from Garden to make model category.

It is this one with wildly differing prices!

https://www.google.com/search?q=Dewalt+DC9096+charger

..

google

metabo sbp 18 plus battery

for online vendors.

2 contact strips and 2 contact strips, so when you plug in the wrong one, the contacts are not making a connection with the correct contact on the drill. Probably just planned obsolescence. Reach out to Black and Decker and complain.

https://www.blackanddecker.com/products/batteries-and-accessories/batteries-and-chargers/batteries/18v-battery/hpb18

A18E

https://www.drill-battery.co.uk/cordless/abatteries.php/6+BLACK_=_DECKER+A18E+129808

The HPB-18 seems to be the newer model and there are complains listed in Reviews of it.

3.6 Ah Replacement Battery Compatible with Black and Decker

https://www.amazon.com > Extended-Replacement-HP...
The description for this battery states it replaces the HPB18-OPE 18v battery. It does not. I have 3 B&D tools that run on that battery and these did not fit ...

How To fix: Zap a Dead Cordless Drill Battery (nicad repair ...

? 5:56
https://www.youtube.com/watch?v=g0JbNX4K4fQ
Apr 27, 2010 - Uploaded by froshmasta
Got an awesome cordless drill with dead nicad battery packs and don't want to throw it out? Maybe a welder ...

Nutool Cordless Drill Kit (18v) x2 BATTERIES - Amazon.co.uk

www.amazon.co.uk > ... > Drills > Drill Sets

Amazon.com, Inc.
Rating: 3 - ‎1 review
Buy Nutool Cordless Drill Kit (18v) x2 BATTERIES at Amazon UK. Free delivery on eligible orders.

18v battery for nutool cordless drill - Fixya

I just went through this with Porter Cable 18V but I can't say it is the same for Ryobi. On the PC, all 18V batteries are interchangable on the tools, but the charging needs are different. A dedicated NiCad charger can't shouldn't be used for liOn but the later chargers will work for either battery.
Ryobi offers an 18V upgrade:
RYOBI Tools

No. The P110 is for NiCad batteries. Any of the Li-Ion chargers like the P113 are backwards compatible and can charge either but do not try to charge Li_ion batteries in a NiCad or NiHi charger.

Do not try this since the charging circuit for lithium batteries are different than nicad! Improper charging of lithum batteries can cause a fire.

For NiCd and NiMH batteries - check the battery voltage (for a good battery it is ~ 19, V); each battery contains 15 units 1.2 V each; you can test each unit after disassembling the battery case (my have 4 screws each, and the case has bottom and top parts). First charge the batteries; second, disassemble cases and check each unit voltage (shouldn't be less than 1.2 V); then take any battery tester (for AA, AAA and other batteries and accus for home usage), attach 2 long (~10 inches) wires to tester's testing contacts, and test each 1.2V unit for current - if it is in "yellow" or "red" zone - the unit is dead and needs replacement

the lithium batteries are not like nicad you can cahrge them whenever you want and will not get a memory. Just put them on the charger and when the ready light is on they are done.

The "memory" effect really only effects nicad and nimh batteries. Lithium is not really affected by this. on early Metabo LI batteries if the batteries had to be reset you had to put the batteries back in the charger. The reset was in the charger. then that was soon changed to having the reset in the battery itself. you could do it by simply pressing the LED button on the back of the battery. It sound like you either have a bad cell in the battery or bad electronics in it.