Backup battery for CPAP machine

Thread Starter


Joined Apr 23, 2020
Hi guys! I wonder if you could give me some guidance for a little project I'm working on. Let me tell you what is about, and if you read something erroneous, absurd or ridiculous, please let me know:

Every night I must use a CPAP machine, a little electric blower that provides pressurized air to a mask. The machine is used to treat sleep apnea symptoms, and I'm not able to sleep without it. As you may have guessed, a power outage means I cannot sleep, and travelling to locations without electricity is a huge no-no for me. For sure, backup batteries for this application already do exist, but they are prohibitively expensive.

Given the power consumption of my machine 12V 3.5A, I got three 12V, 8.5aH AGM, SLA batteries datasheet. My rough calculations supposed that wiring them in series was enough for 7 hours of continuous operation.

Regarding the charing, the manual advises apply constant voltage charge at 2.35v/c – 2.45v/c (14.1 – 14.7v for 12v Monobloc) at 20°C. Initial charging current should be set at less than 2.4Amps. Switch to float charge to avoid overcharging. “Float” or “Stand-By” Service: Apply constant voltage charge of 2.25v/c – 2.30v/c (13.5 to 13.8 volts for 12v Monobloc at 20°C. When held at this voltage, the battery will seek its own current level and maintain itself in a fully charged condition.

So, my workflow was as follows: (question marks designate information I suppose / I don't know)

Phase 1 ChargingPhase 2 Tickle
Apply constant voltage at 14.1 V
Maintain current between 0.85 - 1.7A
Wait until the battery reaches 13.8V??????
Apply a constant voltage of 13.8 V
Given the bad reviews of the OEM charger, I inferred this method wasn't the best, so I kept looking through fishy schematics and blogs (including advertising snake oil charging techniques) and found the DIN 41773 standard, which recommends a three-phase charging method, in order to "get full battery performance".

So I came to this other workflow
C = 8.5 Ah

Phase 1 ChargingPhase 2 Intentional overvoltagePhase 3 Tickle
Keep voltage on 14.1 V
Maintain current between 0.85 - 1.7A
Wait until the battery reaches 14.5v???
Keep voltage on 14.5 V
Maintain current on 0.85 A
Wait until ????
Keep a constant voltage of 13.6V
Even though I still don't have all the information, I started to make basic hardware. I Wired an Arduino to sense the voltage and current of each one of the three batteries, which I guess is the most basic thing to do.

My questions are:

1) Is that three-phase method a real thing? Or should I stick to the manufacturer and forget about it?
2) Should my circuit cut the charging current to the battery, measure the battery voltage, then resume charging?
3) I'm overcomplicating things if I charge all my batteries isolated from each other, and then wire them in parallel to feed the CPAP machine? Is possible to just wire them together and treat them as a single battery? If I do that, how should I adapt either the charging method?

Sorry for the lengthy post guys!

Best regards


PS: I understand that the circuit must disconnect the batteries from the load when they reach a voltage below 10.20 V.
Last edited:


Joined Jun 22, 2012
Three 12V batteries in parallel will give you 25.5Ah , if your machine takes 3.5Amps that's 7 hours approx.

SLA batteries are charged with a Constant Voltage of 13.8v.

Ideally charge them separately for better results.