Don't. SLA's develop sulfate if below a full charge. You want to keep them 100% charged as much as possible. If you forget a night it's not the end of the world, but definitely charge it the next night. I've been maintaining SLA's for 9 years in my wheelchairs
Is it an onboard charger? Can you plug in while at work?
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It's "Dr. _paralyzed_" actually. Not like with a PhD, but Doctor like in Dr. Pepper.