Replies to This Discussion

Permalink Reply by Braith Miller on June 2, 2012 at 11:05pm

I know there are a lot here that run 48 volt Eteck based, assuming that is so far your choice (non-regenerative). 800AH would be my choice on that motor, calculate your weight saved in engine and fuel, most are happy with genset recharging, though fuel weight etc.. Crossing the Pacific I'd be choosing a regenerative drive. Look in links to Solomon's or if there is another competitive plan( if so go for 144 volts). You can pulley rig a 48 volt high amp alternator to the drive line, seems a lot of inefficiency though.  Plenty of excess power when you are on the wind.

Charger specifically, it just raises the time required to charge, just see if the time over double (800) is suitable to you. Ask charger manufacturer about paralleling before doing, I wouldn't. If you want faster find another single high frequency charger.

Please set me straight anyone if there are regenerative drives competitive to Solomon's out yet?

• ▶ Reply

Permalink Reply by Tom Hruby on June 5, 2012 at 2:00am

Hi Roland,

I would like to begin with a word of caution.  You propose to connect your AGM batteries in series as well as parallel to get the power you need. This is a real nightmare in terms of battery management. Both series and parallel banks have their problems and by having both you compound the charging/discharge problems.  

Today’s “smart” chargers that go through 3 phases are designed for cells in series.  They do not address any of charging problems with cells in parallel.  So the simplest solution would be to put a separate charger on each string of 6 volt batteries that add up to your 48 volts. As long as the charger is “isolated” there should be no problem with cross currents.  To be even safer, you can put a diode on the charger wires to eliminate any chance of cross currents. 

Since many golf carts are 48 volts and use the big 200+ amp-hr 6 volt batteries, chargers designed for golf carts will meet your needs.  Your biggest problem will be making sure you have enough shore power.  Each charger will pull between 10-15 amps at 120 volts. If your shore power is limiting you can find smaller 48 volt chargers (e.g. for electric cycles or scooters).

A bank of batteries wired in parallel will “share” the current among the batteries. Due to manufacturing differences, connection quality and other factors, some batteries will receive more current and others less. Neither is a good thing (see below)

1. Parallel connected batteries will try to charge each other to equalize their terminal voltages. If there's a significant difference in the state of charge of the batteries in a parallel connected array, then these inter-battery currents can be high enough to cost quite a bit of service life.  You can avoid this by using diodes to isolate batteries from each other while still allowing them to charge from a common source. If you do this, the battery with the lowest terminal voltage will receive all the charge current and the others will receive none. Once the terminal voltages have equalized, each battery will receive a charging current inversely proportional to its internal resistance. If one of the diode-isolated batteries has a collapsed cell, it will hog all the charge current (because its terminal voltage will never rise as high as that of the others) but at least it won't actually discharge any of the other batteries.

2. Another problem is equal current distribution within the bank when a bank is cycled frequently. Although hard to imagine, even 4/0 cable does have resistance. For current equalization between strings to be the same, all the cables between the charger and the string of batteries must be the same resistance, that is the same length. It is said that one should charge the bank with the (+) and (-) electrodes @ 180 degrees apart. This is said to equalize current flow into the bank by making all the appropriately sized cables, equidistant from the main buss bars to the actual cell terminals (in parallel strings).

Another option would be to consider a system running at a higher voltage.  A 500 amp-hr systen at 48 volts has the same power storage capacity as a 250 amp-hr system running at 96 volts.  This would allow you to have all your batteries in one series only.   A 96 volt system is in the realm of small electric vehicles and there is a wide range of motors, chargers, and controllers for this application you could use in a boat. A 96 volt system will not use any more power than a 48 volts system so your cruising distance will be the same.  Also running at 96 volt system means you can get by with smaller cables for the same amount of power.

 

Tom

P.S. I have a 120 volt system in my electric vehicle that uses 20 of the 6-volts golf cart batteries (230 amp-hrs) so I have about 25 kwatts of stored power, and one charger for the entire bank.

• ▶ Reply

Permalink Reply by Tom Hruby on June 5, 2012 at 2:14am

P.S.  I forgot to mention a critical part of the mutiple charger scenario.   You need to disconnect the strings when charging.

If you use several chargers you will need to put a disconnect switch between the strings of 48 volt batteries.  That is, the connections that make them parallel.  You could use a manual battery switch or rig up a solenoid that breaks the link whenever the charger is plugged in. 

Tom

• ▶ Reply

Permalink Reply by Tom Hruby on June 5, 2012 at 11:23pm

Roland,

Here are some additional thoughts.  To avoid problems with batteries in parallel you might look into getting a 2 volt battery that has the necessary amp hours (400-500) and putting them in series (24 of them to get your 48 volts).  These are available through distributors of solar system.  A 2-volt 500 amp-hr battery weights about 70 lbs which is roughly the same as the other batteries you were considering and would be manageable. You can get 2-volt bateries up to 1000 amp-hrs.  This is the preferred way to get larger capacity banks rather than connecting them in parallel.  A good source of information is the web site for Arizona Wind and Sun but you could find others by Googling "solar power systems".

http://www.solar-electric.com/cosuagmba.html

The solar outfits also have chargers that can charge these larger battery banks.

 

In addition, there is now available a 400 amp-hr lithium iron phosphate battery.  These have a higher initial cost but they last longer and are actually cheaper than AGM batteries.  I did a price comparison and posted the discussion here on May 4 "comparison of life-time cost of batteries."  You might check it out.  The lithium batteries weight about 1/3 of the lead batteries for the same kilowatt-hr capacity so they would be the choice if weight is a problem. Also, each 3.2 volt battery requires its own little battery management electronics so these can be set up in parallel without the problems you have with lead batteries.  The little BMS card is only $15-$20 per battery (<10% of the cost of each battery) and well worth it.

Tom

• ▶ Reply

Permalink Reply by Roland Rodriguez on July 18, 2012 at 11:14am

Hey Tom,

Thanks very much for your information.  It's been immensely helpful.  We're going with an Electric Yacht 360ibl engine.  Given the dimensions of our available space we're going to start off with a 400ah bank of AGMs since I'm finding it hard to find any 6v AGMs with a greater than 400ah capacity.  Coupled with our plans for solar/wind and a small generator as backup we're going to make due with that during our tenure here in PNW for our coastal adventures.  This should give us some baseline data to make a more educated decision towards the end of these batteries' lifetime.  Optimistically in half a decade battery technology will advance and/or prices will drop to afford us additional options for longer passage making.

Thanks again.  I'll keep the group updated with our progress.

Regards,

Roland

• ▶ Reply

Permalink Reply by bill on July 18, 2012 at 8:51pm

hey tom I did not read your post before I made mine. I think this is great advise seeing what he is trying to do.bill

• ▶ Reply

Permalink Reply by Xavi Prats on July 18, 2012 at 2:29pm

Hi,

Did you check Zebra batteries? there are some models at 48Vdc, specially designed for telecom market, completely insolated, easy to charfe with standard chargers, and some advantages:

No memory effect
No autodischarge
Long life (even more than lithium)
100% recyclable
No acid, no explosion risk
No "special" and geopolitical dependant materials as lithium
weight/energy efficiency similar to lithium (not LiPo)

Of course, there is some dissadvantages:

The price isn't low, but financially is good related to the battery life.
There must be some energy source to maintain the internal temperature (about 50W for each 48Vdc pack) that culd be suplied by renewables.

We have a 42 feet boat with this kind of battery for almost 2,5 years without any problems, and there are some vehicles equipped with that batteries.

We are open to explain a little bit more about.

BEst regards,

• ▶ Reply

Permalink Reply by bill on July 18, 2012 at 8:27pm

why don't you look at lithium batteries , you will get more usable amp hrs and faster charge times and less weight. you do not want to bring agm or lead acid banks lower that 50% meaning a 400 amp bank has 200 amp hrs of usable power if you go below that all the time, you will run your batteries into the ground. bill

• ▶ Reply

Permalink Reply by Tom Hruby on July 18, 2012 at 11:47pm

Bill and others,

The Lithium Iron Phosphate batteries would be a good choice if you only rely on shore power for recharging.  Their overall cost per kilowatt-hr "used" is actually lower than that of AGM's over their lifetime.  You can run them to 80% discharge without decreasing their life.   I believe however that they are not really an option if you want to supplement shore power with solar or wind.  The problem is that I have not found any commercially available charge controllers for lithium batteries that would work with solar or wind.  The charging cycle of lithium batteries is quite different and will also require an individual "battery management system" on each cell.  One cannot use the standard chargers that are available for lead acid batteries.  Also, the lithium batteries will completely fail if they are discharged below a certain voltage.  One cannot revive a lithium battery if a mistake is made and they are run down.  That is why it is critical to have to have a battery management system that will shut off the battery if the voltage goes too low.

Also, one can bring the lead acid banks down to 20%, you just reduce the life span of the batteries.  If you look at the specs for lead acid batteries they  often state the number of charging cycles the batteries will last.  The numbers usually reported are for an 80% DOD (depth of discharge). That means a 400 amp hr battery will have put out 320 amp-hrs of power.  A high quality 12 volt deep cycle AGM battery is usually rated at between 400-500 cycles.  A high quality flooded deep cycle golf cart battery is rated at about 800 cycles. The lithium batteries are rated at 1500-2000 cycles assuming you have a high quality battery management system.  

I have done some calculations based on the performance of golf cart batteries I use in my electric truck and found that running lead acid batteries down to 20% charge vs. 50% charge reduces the overall life span of the battery by about 17% - 20%.

 

Tom

• ▶ Reply

Permalink Reply by bill on November 26, 2012 at 6:29am

tom thanks for the great post I have not been on this site for some time now and read this today anyway. I use a honda 2000 when not at the dock. What charger would you go with using the lithium iron 48v set up. I am thinking on them to save on wieght. I can use my agms on my fishing boat and the camper so they will not go waist. bill

• ▶ Reply

Permalink Reply by Tom Hruby on November 27, 2012 at 3:46am

Bill,

The lithium batteries will require a Battery Management System (BMS) to avoid destroying them. Lithiums are much more sensitive to overcharging or dying is drained.  The charger you use will depend on the BMS you chose.  The BMS will turn off the charger if an individual cell goes beyond it limit and will warn you if the voltage in an individual cell goes too low.  I don't have lithiums in my boat because I need the ballast but I have just installed 40 lithium cells (180 amp-hr CALB cells) in my electric vehicle.  I chose the simplest BMS (mini-BMS from CleanPower) and use a basic Constant Current/Constant Voltage charger from Quick Charge.  If you order a charger from them they will program it for lithium batteries given the number of cells you plan to use and their maximum charging voltage. For example the CALB cells have a max voltage of 3.6 while those from Thundersky have a max voltage of 3.9. 

 

The BMS manufacturers generally prefer you to use a simple CC/CV charger rather than a "smart" charger because the BMS becomes the smart part.

Also remember that voltage in lithium batteries does not quite match up with lead acid.  The working voltage on an individual lithium cell is 3.2 - 3.3 volts.  This means that a nominal 48 volts system with 16 cells is actually about 52 volts.  You need to make sure your other electrical components are compatible with this higher voltage.

• ▶ Reply