HI Richard,
Its more than likely that you will be ok, but there are instances when batteries fail within 3 years and you have 8 of them. What kind of batteries do you have? Do you have 2 chargers or 8 chargers in your system. If you send me a diagram of your system I can give it a quick review.
It is possible to run the system on 96 volts but then you would need to change your charger(s) and your controller and add a ground fault monitor and that would be overly expensive.
I have been working on a double diode block housed in an aluminum box that will work for an application like yours. I will finish it off and test it in the early part of January and let you know when it is available. It is quite simple to install with a terminal for each battery, the loads, and the charging sources. The primary battery has to be hooked up directly to the controller and the charging sources, with the auxiliary battery hooked up through the diodes. The reason is the controller and charging sources must be able to see the battery. If there is a diode between the main battery and the controller or the charging sources, these components can't see the battery to know its voltage and the regen function of the controller can't work. Additionally, the controller could fail if there is a diode between it and the battery because there would be no place to put the regen energy and you would likely fry the mofsets or the capacitors.
The simple and safe solution will be the dual battery diode block coming next year.
Happy New Year to all!
James
On Dec 31, 2013, at 4:32 PM, R&M wrote:
James we have 2 48 volt strings in parallel. If I covered to series how could I run a 48 volt system on 96 volts. Currently the batteries are individually charged from shore power and only charged as a string during Regen or trickle charged from 90 watts solar. The interconnecting wires between batteries were kept equal length for equal discharge. Approaching our 3Rd season should we be concerned.RichardSent from Samsung mobile
Robert Lemke <robert-lemke@att.net> wrote:
Nice write up. I've built EVs and off grid power solutions and have always chosen a series string at the voltage and a-hr capacity needed. For off grid it would typically be a 48 VDC bank for feeding the inverter made up of (24) 2 volt cells (700 a-hr to 2400 a-hr). For EVs I have switched to LiFePO4 3.2 volt cells and same for boat housebanks.Bob
From: James Lambden <james@electroprop.com>
To: electricboats@yahoogroups.com
Sent: Tuesday, December 31, 2013 8:23 AM
Subject: [Electric Boats] safety concerns of batteries in parallel
James here, owner of Propulsion Marine and Electroprop.I have read several posts lately about batteries in parallel and there is a real safety issue here.As Santa Barbara's marine electrician for the last 14 years, I have seen at least a dozen failures of batteries in parallel causing thermal issues.This is a common issue as most boats are built today using 12 volts and to increase the ampacity of their battery banks, batteries are installed in parallel. Just because this has been a common practice for years does not make it right or safe.The issue is when the battery gets an internal short in one of the cells. The short is then driven by the other battery held in parallel. It only takes about 10 amps to drive a short to the point of a thermal so it is not possible to protect the battery with a fuse. The issue happens most commonly at the end of a batteries life.A lead acid battery can get so hot that it catches fire or explodes. This I have personally witnessed as I have been there to diffuse the situation several times and believe me it can be very scary.Typical chargers do not have thermistors for all of the batteries and are not monitoring for the thermal condition on each individual battery.If you have batteries in parallel, then be sure to keep them out of parallel when the boat is not being monitored. Batteries in parallel should be charged independently. If you are hooking up one charger to multiple batteries, the second battery should be hooked through a diode block for the charging, and a second diode block for the loads. There is a 1/2 volt drop in the diode block so it must be heat sinked. On 48 to 96 volt battery banks, this half volt drop is not significant enough to cause a charging problem. Set the charger up on the main battery at 1/4 volt more than recommended and the auxiliary battery will then function at 1/4 volt less than recommended. On a 48 volt battery this works out to .125 volt difference in charging at the 12 volt battery level.When recombining batteries in parallel to operate your boat be sure that the battery banks are of a similar voltage or the switch can handle the current that will flow from a fully charged battery to a fully depleted battery. If the batteries are of a dissimilar voltage then either charge up the low battery, or put a load on the high battery while you combine them.It doesn't matter which way you hook the batteries up, there is an issue every way you can hook them up.If you want a bigger battery bank use a 6 volt or a 2 volt cell - Lifeline batteries have great 2 volt, 1200 amp hour batteries.It is ok to hook cells up in parallel and we see this done frequently with Lithium batteries. Cells will work well in parallel because they don't have the higher potential to drive a short. If you look inside a cell, the plates are hooked in parallel anyway, so joining another cell in parallel is fine. You could also safey hook up 2 volt batteries in parallel because a 2 volt Lead Acid battery is a cell.Building a bigger battery bank can also be accomplished by increasing the voltage of the system so the batteries remain in series. System voltages up to 100 volts are still very safe. Just be sure to add a ground fault monitor on both the positive and negative of the battery bank, and keep the battery floating (not hooked to ground)The great thing about electric boats is we can build big batteries in series and avoid the parallel issues that are troublesome and dangerous on a typical 12 volt diesel boat. This is one more reason why switching to electric can make your boat safer.JamesJames LambdenThe Electric Propeller Company625C East Haley Street,Santa Barbara, CA93103805 455 8444jlambden: Skype
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