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By Mee_n_Mac
#153141
As I read the LTC4412 datasheet, the Vin (Vcc ?) is the battery voltage. It gets switched (or not) to the load by an external FET which is controlled, on or off, by the Gate signal. The Sense signal is connected after the FET (on the load side) and whenever it sees a voltage that's >20 mV above the Vin level, it turns the FET off. The Stat signal is an open drain that can be used to let other circuitry know the state of the Gate signal.

So as I understand it, yes it's for switching between a battery and another source. You still need a low voltage cutoff. You might be able to use this (2 of them) in place of the steering diodes that your design (above) has to connect the 2 LiPos. But it's tricky. You've set yourself for a more complicated situation than your premise of "a circuit that functions similar to what a laptop would do: when it's plugged in, it charges the battery, and when unplugged it runs off of battery". By requiring that you have redundant batteries, you now require each battery to have enough capacity to run everything on it's own (for some period of time). And because you might hot swap a dead or failed battery, you can't just wire the 2 LiPos in parallel because their state of charge (SOC) might be drastically different (even if they're otherwise matched batteries). That then means you have to have 2 separate charging circuits for the 2 separate LiPos. Which then means 2 diodes to combine the LiPo outputs ... or 2 of these ... maybe. I'm not sure how these would work. I'd guess that tying the load to the output side of the 2 FETs would mean that only 1 LiPo is running everything at any one time, until the SOCs equalize. And then some load sharing might happen, or perhaps the LTC4412's will be switching back and forth as the current draw through each LiPos internal resistance changes their respective output voltages. I dunno. :?

I have a failing memory that some one-IC-to-rule-them-all LiPo switching/charging/cutoff IC was previously discussed in this forum. Alas I can't recall the thread or part number. I wonder if the LTC1479 might be that part ?

The LTC®1479 is the “heart” of a total power management solution for single and dual battery notebook computers
and other portable equipment. The LTC1479 directs power from up to two battery packs and a DC power source to the
input of the main system switching regulator. It works in concert with related LTC power management products
(e.g. LTC1435, LT®1511, etc.) to create a total system solution; starting from the batteries and the DC power
source, and ending at the input of each of the computer’s complex loads. A system-provided power management
mP monitors and actively directs the LTC1479.

The LTC1479 uses low loss N-channel MOSFET switches to direct power from three main sources. An adaptive
current limiting scheme reduces capacitor and battery inrush current by controlling the gates of the MOSFET
switches during transitions. The LTC1479 interfaces directly to the LT1510, LT1511 and LT1620/LTC1435 battery
charging circuits.

Battery Voltage Monitoring
The LTC1479 has the ability to independently monitor both battery packs. (Because of this, one battery pack may be
discharged as the other is being charged.) A low-battery detector signals when the selected battery pack has dropped to the level where a shutdown sequence should be initiated or the other battery pack engaged.

(click on to enlarge)
LTC1479.jpg
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By kirash4
#153153
Unfortunately, the LTC1479 says battery operating range to be 6V to 28V. A LiPo is 3.7V ... And it doesn't mention support for LiPos, only Li-Ion.
By kirash4
#153165
Someone please explain something to me. I was always told that if you have a multi-cell LiPo or Li-Ion pack, you must use a charger that can balance the cells when charging. However, I see a lot of multi-cell packs that only have 2 leads and some that have those 2 leads and the mention of a 'protection PCB to protect against over-charge or over-discharge'. And then I also see battery packs that have an additional tail that gets connected to a balancing charger.

So for those that are simply two leads, how are you supposed to balance the cells? I mean, the reason I'm staying away from multi-cell packs is so I don't have to deal with balancing the individual cells. But if these packs that just have the two leads work just as well, then what am I missing here? Why wouldn't they too need a balancer?
By Mee_n_Mac
#153192
Are those packs serial or parallel ? I'd think the cells in parallel packs are all at the same SOC by virtue of the wiring. The only reason for balancing those cells would be to account for differences as they age.

Series packs should have tabs to enable balancing. If they don't ... well you get what you pay for.
By Mee_n_Mac
#153195
Apparently not uncommon. I'd expect the cells to eventually become unbalanced enough that you'll not get the combo optimally charged and so the runtime and lifetime would be reduced. The question is how long will it be before the cells become too unbalanced. Some people have good luck and aren't worried about 2S packs. Others are less "adventurous". See below.

http://www.rcgroups.com/forums/showthread.php?t=1312368
http://www.rcgroups.com/forums/showthread.php?t=329280
http://www.ultimaterc.com/forums/archiv ... 23895.html
By kirash4
#153196
Well, that means I'm back to single cell packs with individual charging circuits each. Unfortunately, the LTC solution you mentioned above require a battery voltage range of 6V to 28V. Single cell LiPo will only provide 3.7V. Unless I use 4 of them, with two series pairs and then tie that in parallel.
By kirash4
#153610
Ok, so I'm still trying to come to a solution for this. I would love to use the LTC1479 simply because it can handle the two batteries and all related aspect of charging and switching between them and external power. The problem is that it states battery operating range to be between 6V and 28V. But a single cell LiPo (or Li-Ion) only supplies 3.7V. What am I missing here?