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I want to try my hand at producing a POE battery power supply for use with an access point [AP] for site surveying purposes.

My solution is an 11.1v LiPo 3s pack and charger fro a remote control car rated at 5000mah and a 12V --> 48V step up converter from Ebay, connect the two together and then connect the 48V to a RJ45 Jack and hey presto portable power for the AP.

Questions -

1) Given a rated power consumption of 200mw what life would I get from the battery? 2) As the POE standard allows for some negotiation on the power supply would simply presenting 48V to the appropriate pins of the Cat5 wirk or do I need some logic in there? 3) Am I likley to hurt my AP with this solution?

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PoE (Power Over Ethernet) isn't as simple as just providing 48V. There is some analog signalling that goes back and forth. A PoE power supply isn't allowed to provide the 48V until some negotion has occurred. On the device end, it might just work if 48V is always applied, assuming the device was designed to handle that (is PoE capable). However, that is outside the PoE standard so it is not guaranteed.

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  • \$\begingroup\$ Yes. The standard specifies negotiation, but many non-standard "passive POE" systems simply drive +12V or +48V on the appropriate pins of the Cat5 cable; that is unlikely to damage a PoE-capable access point. \$\endgroup\$ – davidcary Oct 27 '11 at 15:11
  • \$\begingroup\$ @davidcary: One reason for the negotiation is so that PoE switches don't damage devices that don't to PoE and therefore might be hurt by 48V. Simply applying 48V is very unlikely to damage a PoE complient device. However, depending on the PoE control logic in the device, it might not enable its power without negotiation. That said, the PoE chips I'm familiar with enable power whenever they sense the 48V whether it was preceeded by negotiation or not. In other words, what the OP wants to do will probably work, but is not guaranteed. \$\endgroup\$ – Olin Lathrop Oct 27 '11 at 16:13
  • \$\begingroup\$ There are plenty of 100Mb devices which will connect 4,5,7,8 together with low value resistors, and there are plenty of devices which need 24V, not 48V. That said from a quick read of 802.3af I can't see an 802.3af device being damaged by having power applied. \$\endgroup\$ – james Feb 11 at 15:37

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