John, when your batteries are under a heavy load, there will naturally be a big voltage drop. If you want to estimate your State Of Charge from battery voltage, you need to let them rest for at least 12 hours with zero load. Then look at the voltage. But there is more. You can calculate how many hours of run time you have available at the Peukert-neutral discharge rate, or in plain English the 20 hour discharge rate. Look at the rated capacity, divide by 20. So fir say a 220ah bank, 11a. Now run for 10 hours to achieve approximately 50% SOC. Rest the batteries 24 hours and check SOC by voltage or by SG. Compare your findings to the predicted 50% SOC and calculate the ACTUAL time required to discharge your batteries to 50%, then perform the same test again. From your findings you can calculate how many hours required to discharge to 50% with reasonable accuracy at the 20 hour discharge rate.
Okay take a break now. Read that again.
So now you can know, from fully charged batteries, your 50% range, but only at the 20 hour discharge rate. However, that doesn't mean you can run 22a for 5 hours and get 50%, or run 110a for 1hr and get 50%, no. Enter the Peukert Effect.
The higher the discharge rate, the less total power you have available in your bank. You can calculate as if Peukert didn't exist and at high amperage you would take your batteries down to maybe 25%!!! And you wouldn't even realize it fully until you rested the batteries and tested them! Conversely, if you run at 3a, you would calculate your run time for 50% SOC and run for that duration and maybe end up with a 65% or better SOC. So it is not linear. To calculate actual usage and remaining charge you need to apply the Peukert Coefficient. You can calculate this number by running at say 1/5C (in our hypothetical example, 44a) and crunching the numbers. There are online sources for this formula and maybe I will make another post on this but there is a spreadsheet for calculating Peukert and then applying Peukert to find SOC on my website, http://www.growleymonster.com/eboat/index.html.
Anyway, if you monitor current and elapsed time, you can calculate remaining charge with a fair degree of accuracy. There are gauges that will do this for you. Some better than others. There is one called the Smart Gauge that is pretty good. They all must be programmed for your bank or the results fudged to match your operating parameters. These gas tank gauges can be useful at a glance indicators and save you a lot of calculating, though accuracy can be very dodgy.
The good news is you will not trash your batteries by occasionally dipping down to 40% or even 30% if they are true deep cycle FLA batteries such as good old golf cart batts. So if your calculations and smart gauges are not spot on, it's not the end of the world. Yes, at deeper discharges, you reduce the total number of charge/discharge cycles. However at deeper discharges you are also getting more kw/hrs out of the batteries for each cycle, and so actually you can end up getting the same amount of actual use out of them at deeper than 50% discharge, or possibly even better. In fact, an annual discharge to 0% can yield useful data, giving you an actual capacity that you can compare to the previous year, and this will not significantly harm the batteries. When the capacity is found to be down to 80% of new capacity, you can start saving your nickels for a new set because those are soon going to need replacing.
If you want to REALLY make this complicated, get a portable 2kw gasoline generator such as the popular Honda or my favorite, Ryobi. Use a Variac to reduce the output voltage and then a rectifier from an old welding machine to make a very ripply DC out of the reduced AC, and feed it to the batteries, monitoring controller supply current and rectifier output current, and system voltage, to keep yourself in the zone. Plugging a "smart charger" into the generator and charging on the go, so to speak, can have unpredictable results. Or just ignore it and keep system voltage up around 49.5v while you motor home, by adjusting the Variac. Note that a Variac and a tangle of cables outside on deck can be very unsafe in wet conditions!!!! Consult an electrician if you are not very very good with electrical stuff. The portable gen is a very inefficient way to make your e-boat go. However, it is a very good way to get yourself home when your batteries are low, too.
So the simple answer is to run your boat at C/20 and mathematically calculate how much juice you have used and how much remains. For different discharge rates you need to get into Peukert calculations.
Note that this only applies to FLA batteries.
Oops I see you have AGM. Different animal but most of that applies, still, except for the annual capacity check. Rest the batteries 24 hours for an accurate SOC reading.
---In electricboats@yahoogroups.com, <jack21322@...> wrote :
Can someone help me out on how to read the amp and and volt meter in relation to the range of the battery bank?
I'm running a 10kw thunderstruck 2:1 gear reduction on my erickson 29 with a 48v bank of 4 group31 agms. It works great for the way I sail. I only have to throttle up slightly to go the speed I like to go down the channel. When I engage the motor the voltage usually drops from around 52 to about 48.9, once the sails are up and the motor is off the voltage goes back up to around 52. I figure the 48.9 means how much juice I have at that speed? What voltage should I stop motoring to prevent damaging the batteries? I know it's not good to take them below 50%. If I throttle higher the voltage drops more. I don't know much about electricity so if anyone is willing to explain this to me like I'm a five year old I would be grateful.
Best,
~Jack
P.s. does anyone have any easy to understand reading or youtube recommendations for electric drive and battery banks?
On Mon, Aug 27, 2018, 9:45 PM Daniel Michaels nov32394@... [electricboats] <electricboats@yahoogroups.com> wrote:Your amps are going to be the same even at low rooms. Think of amps as power to turn something. It does not matter how fast it goes it is how hard it is to turn.
On Mon, Aug 27, 2018 at 7:16 AM, 'Gennaro Fazio' gfazio@... [electricboats]<electricboats@yahoogroups.com> wrote:I installed a 10kW Thunderstruck system with a 2:1 gear reduction and 275 Amp Hours 48 V AGM batteries on my 1979 Catalina 30, original 2 blade propeller. I can get to 5 knots with 56 Amps – max throttle - I don't do that often. I usually like running at 3 knots at 10-15 Amps. Depending on wind conditions. I am thinking that if I switch to a three blade propeller I might get a bit more push at low RPMs. VERY HAPPY with the conversion. Gennaro
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
Sent: Thursday, August 23, 2018 1:25 PM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: My eboat ready to launch
I own a 1989 Mirage 29, which currently has an original Volvo Penta 2002 diesel motor. I am considering converting to electric - the old motor won't last much longer, and the admiral hates the smell!
I have looked at the 10kW Thunderstruck motor - it seems to be a good match. I am struggling with battery capacity, however. If I wanted an endurance of, say, 5 hours at 5 kts, what would I need? I would consider LiFePO4 batteries - expensive, but apparently long lasting.
Any comments or guidance you all could provide would be much appreciated..
Dave
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