T400s 6 cell battery compatible with T420s? Cheap option?

[ashirus]

I'm looking for a new 6 cell (or 9 cell) battery for my T420s as a backup for long journeys.

I'm happy to get the normal slot-in battery, not an ultrabay as they are cheaper and I'll just put my thinkpad into hibernate while I change batteries.

The official Lenovo batteries page says the code for T400s battery is 51J0497 but for T420s is 0A36309.

I can pick up a T400s compatible-one quite cheap but the T420s ones seem quite expensive.

some shaooping sites appear to say the 51J0497 can be used with a T420s, but the Lenovo compatibility grid doesn't mention this.

So, are the t400s-compatible ones usable on the T420s and I can grab a bargain?

Failing this, are the generic compatible/imitations ones likely to be a big risk?

(It's a US model of T420s but I use it in the UK).