I bought one of these (3 stage switch mode) some time ago, been used at Astrofest to keep my battery and equipment running, works well and only $38. I've bought a couple more for my workmates, all have performed without problems and do the job as advertised. I monitored my SLA battery on charge and the charger nicely terminated the charge and trickled to maintain the battery as advertised.
http://www.ebay.com.au/itm/NEW-12V-B...item41773f344c