I was pretty satisfied with the Mikita 36v(two 18v batteries in series) self propelled 21" walk behind mower. For $499 it came with 4 5ah batts and held all 4 but you'd have to physically take 2 out and replace them with the onboard spares. Mikita also makes one where all 4 batteries are in use, 2 parallel 2 series for again 36v but that one has about 20 min less runtime total, I believe it's because it's more the professional model and has a steel deck and probably more power than the $200 less priced one. All in all while not as powerful as a ICE version, it was perfectly usable for a normal-sized yard and you didn't let the grass get real high or try and mow when the grass was wet or very thick. In those cases the pro model would probably be a better choice. Both models came with the same sized 5ah batteries and 4 of them. The more expensive model may have come with a faster dual charger too, that or it's an option to purchase,