need 12v from 56v

[Ultra]

I have 2 eGO batteries, 56V; 1 is 7.5 AH (420W) and 2 is 5 AH (280W).

I have a CPAP machine that plugs into a AC outlet and w/ the inline transformer the unit uses 12 V @ 6.67A so about 80 W (usage depends on settings, etc)

Looked around on how to step down the 56V to 12V but not finding an obvious solution. (I did see someone use a 48V step-down transfer to 12V. someone else tried and it killed their battery).

eGO has an inverter; 120V AC - 150W output - max. While this seems to add extra steps to the process, DC to AC to DC (using the CPAP cord) it seems simple. I'm wondering how much time I can get on the two batteries, the 7.5 and the 5 for the 12 v CPAP @ 6.67A / 80W. I understand I'm using power to convert from DC to AC back to DC....

https://egopowerplus.com/battery-7-5amp/
https://egopowerplus.com/nexus-escape/

I'm sure the answer is staring me in the face, but my math w/ DC is not so good. Any help with this or other ideas would be greatly appreciated! Simple is easier for me. The inverter is $100 while a specific 12v/24v CPAP battery is $300. I like the idea of using the eGO batteries I already have.

 

[bertus]

Hello,

The closest I can think of is a 150V Low IQ, Synchronous Step-Down DC/DC Controller with the LTC7801:
https://www.analog.com/en/products/ltc7801.html#

Bertus

 

[Ultra]

thanks bertus. i found a picture of it which leads me to more questions:

1. is the ground simply to the case and the rest of the neg terminals? i would quess yes and that it does not a ground to earth.
2. i don't see any leads. how to connect in and out? perhaps the ports are on the sides?
3. how do you adjust the voltage settings?
4. does it need a heat sink or fan?

thanks!

 

[dave9]

You did not provide your location, important for purchasing products (sometimes) !

I went on ebay and searched "Buck regulator 10A". 10A to give a little current margin over the 6.67A spec you provided, which itself might be conservative, but then again you can decide for yourself if you want to be even MORE conservative with *generic* Chinese products. The 2nd link below might be a better choice given the application.

https://www.ebay.com/itm/Waterproof...-42V-60-V-72V-to-DC-12V-120W-10A/222860367233
https://www.ebay.com/itm/223130887152

One catch is that ego uses nonsense battery pack ratings, apparently their "56V" depends on each cell being 4.0V so it is 14 cells in series, then a 2nd parallel series for 5.0Ah or 3rd parallel series for 7.5Ah. This means with 14 cells (in series), if the linked products above really can't go any lower than 48V input (which is doubtful because that's such a nice, round, common number, rather it can probably go lower than that but you might have to test this if nobody else has yet), then at 48V input the series of 14 cells only drops to an average of 3.4V/cell so you wouldn't get anywhere near full capacity out of them.

Those buck converters above might work much lower than 48V input, even down to 40V input working would get you down to avg. of 2.85V/cell under load, "good enough", except you have to decide if buying a generic Chinese product is "good enough" for this application.

I did see someone use a 48V step-down transfer to 12V. someone else tried and it killed their battery)

The details of why it killed their battery "could" be important, including exactly what "set-down transfer is". Is that a buck regulator or a linear transformer or something else?

Do ego tools have the low voltage cutoff in the tool rather than the battery pack? I'd think they have to have one in the pack for safety reasons, but ultimately you would have to set up a test rig to monitor real power consumption and state of battery charge after the needed run period. Transformer rated at 6.67A, will not mean it consumes that continuous all night long, could only be a small fraction of that on average.

If the buck converter was undersized and self destructed from heat, that too could harm a battery. One advantage with a larger converter product casing and 12V output, is it might be easy to cut a hole in the housing and vent exhaust holes at the opposite side, and strap on a low RPM (quiet) 12V computer fan to reduce operating temperature. I may be getting ahead of myself and this isn't needed, or on the pic of the larger current model it appears that at least "some" of the switching transistors are heatsunk to the casing itself so it might be enough like that for continuous operation. It's "supposed' to be if rated correctly, but you never know on cheap products until they prove it. You can monitor temperature during testing.

There's bound to be higher quality, more industrial oriented major brand buck switching converters that would work, but probably costing an order of magnitude more money.