Zivan and the like are probably just quoting the efficiency of power out to power in of their chargers at their best operating point ( probably in bulk charge). the "charging efficiency " is a characteristic of the battery itself. So even with a magical 100% efficient charger you'd still need about 20-30% more power into a lead acid than you would get out.
Chris S
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Pat,
>
> I agree with your comments about charger efficiency completely. I was hoping that somebody would read my post carefully enough to see that discrepancy. Still, regular chargers like Zivan publish their charger efficiency as >85% and PFC chargers like Elcon and others claim >92%. While the difference is half as dramatic as I stated before, the difference in efficiency is still noticable. In practice, PFC chargers deliver more amps for the same RMS rating than more traditional chargers. I also believe that underestimating performance sets more realistic expectations. ;)
>
> I don't think that the charging efficiencies due to voltage differences are directly related to Puekert Effect. You can see this by drawing down the batteries at the rated 20 hour rate (10A for 200Ah). There is no decrease in capacity from Peukert, but the difference between Wh in versus Wh out remains, due to the charging voltage spread. The Peukert calculations start with the same resting capacities for all battery chemistries, the formula only factors amps, there is no reference to voltage. The formula works regardless of the pack voltage. I think that this is why SOC meters focus on Ah instead of Wh when tracking the current in and out, one less variable to average out.
>
> But again, you may have caught a place where I have exaggerated the net effect; the charging efficiency should have been between the charging voltage and resting voltage, not the discharge voltage. This should isolate the charging and discharge efficiencies. So for Lithium and FLA, the charging efficiency for 12V should be 12.8V divided by 15V or 85%, and AGM should be 12.8V divided by 14.4V or 89%. There is still more complications in this calculation because much of the charging takes place at a rising voltage during the bulk charging phase, but I stack that smaller difference into the underestimation of performance to cover other random inefficiencies in the system like wiring and connection resistances.
>
> I'm not an EE, nor do I play one on TV, so if anyone has more to teach me about these topics, I'm all ears.
>
> Thank you for diving deeper into this topic.
>
> Eric
>
> --- In electricboats@yahoogroups.com, "greenpjs04" <greenpjs@> wrote:
> >
> > Hi Eric,
> > Your post gives a great summary of all the factors that affect charging efficiency, but a few of the details aren't quite right.
> >
> > When comparing the input watts to the output watts of a charger, you are using the amps times volts going into the charger as its input power. This isn't quite right for two reasons. 1) the amps on the nameplate or in the specs is the maximum current that can be drawn. The average is often much lower. 2) Power factor. Power supplies often draw their current slightly out of phase with the voltage. This reduces the actual power used. Power factor isn't much of a factor (no pun intended) for battery chargers, but not drawing the max amps all the time is. It probably explains the discrepancy between the manufacturer's and your efficiency calculations.
> >
> > Next, I like the way you discuss battery efficiency by comparing input voltage during charging to output voltage during discharge. I hadn't thought about it that way before, but it makes sense. However, if you do that, you can't then add the Peukert effect. Peukert shows up to the outside world as a lower voltage during discharge so you actually counted it twice.
> >
> > Pat
> >
> > --- In electricboats@yahoogroups.com, "Eric" <ewdysar@> wrote:
> > >
> > > While we're talking about overall efficiency from external power source to the motor controller, there are three major areas that should be considered; the ability of the charger to turn AC into DC (charger efficiency), the ability of the battery to store the provided DC (charging efficiency) and the ability of the batteries to deliver the stored energy to the motor controller (battery efficiency).
> > >
> > > First, let's look at charger efficiency. The easiest way to evaluate a charger is to divide the watts coming out by the watts going in. Zivan chargers are common and according to their published specs operate at 85% efficiency, but the NG1 draws 12A @ 115VAC (1380W) and has a stated max output of 900W. This calculates to about 65% overall efficiency. The NG3 does better, with 18A @ 115Vac (2070W) to 1500W output or 72% overall efficiency. While Dual Pro doesn't publish their current requirements, I believe that they operate much like the Zivan chargers in technology and therefore, efficiency. Alternatively using a newer technology, PFC (power factor corrected) chargers like Delta-Q, Elcon, and Manzanita have overall efficiencies above 85%. My Elcon 2000+ will push 1400W from 14A @ 115VAC (1610W) or 87%.
> > >
> > > Now, let's look at the batteries. Focusing only on the question of KWh in to KWh out, one of the obvious places to look is the charging voltage to discharge voltage. For this part of the discussion, I'm only talking about the current coming from the charger and the current going to the controller.
> > >
> > > Assuming the amp hours are similar going in and going out, the difference is in the voltages. The max charging voltage for my 48V nominal Lithium battery pack is 60.8V, the resting voltage is 52.8 and the discharge voltage averages around 50V. In simplistic terms, the 9700Wh pushed into the batteries, stores at 8450Wh and will provide 8000Wh to the controller. This works out to about 80-85% charging efficiency for lithium batteries and flooded cells (FLA). If you consider AGMs with 14.4V in and 12.5V out, their charging efficiency is almost 87%.
> > >
> > > So far, we've been assuming all amp hours in are available as amp hours out. There are other factors, the most apparent is the Peukert Effect, the energy lost due to the battery chemistry's inability to handle high amp loads. FLA suffer the most, AGM much less and Lithium very little. If we use an average load of 60A against a 200Ah bank, most FLA batteries will only deliver less than 60% of their rated capacity to 100% depth of discharge, AGM can deliver about 83% of their rated capacity and Lithium more than 96% of their rated capacity. Smaller packs with lower Ah ratings exaggerate these differences.
> > >
> > > So the worst case scenario is FLA with an old charger or 70% (charger) times 80% (charging) times 60% (battery) where only 34% of the energy provided to the charger is available to the motor controller while under way. Just switching to a PFC charger will up that to 41% (85 x 80 x 60). Moving to AGM (85 x 87 x 83), they're operating around 61% total efficiency from outlet to motor controller. Finally, Lithium with a PFC charger (85 x 80 x 96) delivers just above 65% of the watts going into the charger to the controller during normal operation.
> > >
> > > If you are running the charger from a generator while underway, you get to skip the charging and battery efficiency and focus entirely on the charger efficiency. This still ranges from 65% to 85%, so your results will depend on which charger you choose.
> > >
> > > Fair winds,
> > > Eric
> > > Marina del Rey, CA
> > >
>
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