PD or Iota for my upgrade?

BFL13 wrote:
The Iota with the IQ4 does bulk at 14.8 until battery reaches 14.6 (the "trigger") then after another 15 minutes it drops to 14.2 Vabs.

Hmmmmmm. I don't understand what it means to say that the IQ4 is doing 14.8 until the battery reaches 14.6. I did a quick search and didn't see anything saying the Iota has a remote battery voltage sensor, so we can't be talking about cable losses.

In the absence of that sensor, a typical charge profile will have the charger increase its voltage until it reaches a current limit. With the battery discharged, the current limit will be reached long before the charger voltage reaches 14.8. For example, with my PD, it takes about 3 hours from 50% SOC on the 460AH bank before the charger has to go to 14.8 to push in current at the current limit. During that entire time, the charger (and battery) are below 14.8 volts.

So what does it mean to say that the Iota does bulk at 14.8? If the Iota charger is really at 14.8, then the charge current will be well above 80A (at least on my batteries). That would burn something out if the charger isn't rated above 80A. If it's not really at 14.8, and it's at some lesser voltage to limit the current, and only charges until the battery reaches 14.6, then the charger would never reach 14.8 ???

Didn't I read that the Iota always starts in bulk mode (perhaps at 14.8 regardless of the battery voltage) and stays there for at least 15 minutes? If that's the case, are you saying it will go as high as 14.8 in that first 15 min window, then drop out if the voltage is 14.6 or above?

(PowerMax chooses to drop to 13.6 for Vabs instead which is even more "gentle." That's why I got in there and changed it to 14.6 (now at 14.8 recent change) on mine that has the internal voltage pot.

Aha - I see what you are talking about. You are referring to the voltage of the charger when it enters the longer term final charging mode (not the float/maintenance mode, but the actual final charging). I've seen it referred to as the "absorption" mode, the "topping" voltage/mode and (what PD calls it) the "normal" mode. PD uses 13.6v for that mode.

I have not modified that voltage, although I could. I was questioning what happens at the point where the battery has reached the output voltage of the charger, which is not necessarily the point at which the absorption mode should be entered.

Of course it depends on your charging rate what the SOC is when that happens.

Yes. And it's really SOC that determines when that mode should be entered, which can only accurately be determined from the specific gravity. Since our chargers can't measure that, they use another criterion. Most seem to use time or current as a proxy. If the current drops sufficiently and/or the charger has been in boost/bulk for long enough, the charger's preset charge profile will cause it to enter absorption/topping/normal mode.

From the reading I have done, the ideal charger would use constant current in the bulk mode, no matter how high the voltage rose. It would stay at constant current until it reached the desired SOC (as determined by the SG) and then it would switch to constant voltage for the topping/absorption, followed much later by float mode.

No chargers do this, since they have no way of knowing the true SG and SOC and thus, no way to know when to switch out of the constant current mode into the absorption mode. This isn't really a problem until the voltage at the battery has reached 14.4 or so, However, above that voltage, although it may be perfectly OK to supply the same current, it may not be OK. One can only tell from the SG/SOC, which is info not available to the charger.