Some brands put the protection in the battery (Milwaukee 20v does) others put it in the tool (EGO, Dewalt). If it's in the battery you're probably fine. If it's in the tool and you're using a differnt tool, a cheap hack would be to put a little voltage meter on it, and know what voltage you should not drop below. (I think it's 2.5v per cell on li-ion.)

edit: If you're talking about putting battery packs in series, (2x 20v packs for a 36/40 volt tool for example) an ideal diode might work: https://product.torexsemi.com/en/technical-support/techfaq/d...